UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant  x                            Filed by a Party other than the Registrant  ¨

Check the appropriate box:

WILLIAM BLAIR FUNDS

222 WEST ADAMS STREET

CHICAGO, ILLINOIS 60606

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD AUGUST 2, 2012

June 27, 2012

To the Shareholders:

You are invited to attend a special meeting of the shareholders of William Blair Funds (the “Trust”) to be held in the 2nd Floor Conference Room at 222 West Adams Street, Chicago, Illinois, 60606, on Thursday, August 2, 2012 at 2:00 p.m. Central time, to consider and vote on the election of eight (8) Trustees to the Board of Trustees of the Trust and to transact such other business, if any, as may properly come before the meeting.

The Board of Trustees of the Trust has fixed the close of business on June 4, 2012 as the record date for determining the shareholders of the Trust entitled to notice of and to vote at the meeting or any adjournments or postponements thereof. Shareholders are entitled to one vote for each share held and a proportionate fractional vote for each fractional share held.

In order to avoid delay and additional expense and to assure that your shares are represented, please vote as promptly as possible, regardless of whether or not you plan to attend the meeting. You may vote by mail, telephone or over the Internet. To vote by mail, please mark, sign, date and mail the enclosed proxy card. No postage is required if mailed in the United States. To vote by telephone, please call the toll-free number located on your proxy card and follow the recorded instructions, using your proxy card as a guide. To vote over the Internet, go to the Internet address provided on your proxy card and follow the instructions, using your proxy card as a guide.

PROXY STATEMENT

WILLIAM BLAIR FUNDS

222 WEST ADAMS STREET

CHICAGO, ILLINOIS 60606

June 27, 2012

Important Notice Regarding the Availability of Proxy Materials for the Shareholder Meeting to be Held on August 2, 2012:

The Proxy Materials are available at: www.eproxy.com/williamblair

GENERAL

The accompanying proxy is solicited by the Board of William Blair Funds (the “Board” and each member thereof a “Trustee”) for voting at the special meeting of shareholders of William Blair Funds (the “Trust”) to be held on August 2, 2012 at 2:00 p.m. Central time in the 2nd Floor Conference Room at 222 West Adams Street, Chicago, Illinois, 60606, and at any and all adjournments thereof (the “Meeting”). This proxy statement and the enclosed proxy are first being mailed to shareholders on or about June 29, 2012.

Shareholders are being asked to consider and vote on the election of eight (8) Trustees to the Board of Trustees of the Trust (the “Proposal”) and to transact such other business, if any, as may properly come before the Meeting.

Shareholders of each Fund will vote together as a single class to elect Trustees to the Board. A quorum of shareholders is required to take action at the Meeting. Holders of one-third of the shares of the Trust represented in person or by proxy, will constitute a quorum of shareholders at the Meeting. If a quorum is present at the Meeting: (1) the affirmative vote of more than 50% of the shares of the Trust present at the Meeting, either in person or by proxy, will be required to re-elect each of the current Trustees to the Board; and (2) the affirmative vote of a plurality of the shares of the Trust entitled to vote at the Meeting will be required to elect the Nominee to fill the current vacancy on the Board of Trustees. Abstentions and “broker non-votes” (i.e., shares held by brokers or nominees as to which (i) instructions have not been received from the beneficial owners or the persons entitled to vote, and (ii) the broker or nominee does not have the discretionary voting power on a particular matter) will be treated as shares present at the meeting for quorum and voting purposes. Therefore, abstentions and broker non-votes will have the effect of a negative vote for purposes of the election of the current Trustees and will have no effect for purposes of the election of the Nominee to fill the current Board vacancy.

Each valid proxy will be voted in accordance with the instructions provided on the proxy and as the persons named in the proxy determine on such other business as may come before the Meeting. If no instructions are given, the proxy will be voted FOR the election of the Trustees. Shareholders are entitled to one vote for each share held and a proportionate fractional vote for each fractional share held. Shareholders may vote by filling out and signing the enclosed proxy card and returning it in the postage paid envelope provided. Shareholders may also vote by telephone or over the Internet. Please see the instructions on your proxy card for telephone voting and Internet voting. Shareholders will have an opportunity to review their voting instructions and make any necessary changes before submitting their voting instructions and terminating their telephone call or Internet link. Shareholders who execute proxies in any of the above manners may revoke the proxies at any time before they are voted, either by sending the Trust a written notice of revocation, by delivering a duly executed proxy bearing a later date, or by attending the Meeting and voting in person. For directions on how to attend the Meeting in person, please call 855-844-8646.

The Board has fixed the close of business on June 4, 2012 as the record date (the “Record Date”) for the determination of shareholders entitled to notice of and to vote at the Meeting. As of the Record Date, 1,940,781,527 shares of the Trust were issued and outstanding in the following series and classes:

2

PROPOSAL

ELECTION OF TRUSTEES TO THE BOARD

(All Funds)

General

At the Meeting, shareholders of the Trust will be asked to elect eight individuals to constitute the Board of the Trust. The Board has nominated Vann A. Avedisian, Phillip O. Peterson, Lisa A. Pollina, Donald J. Reaves, Donald L. Seeley, Michelle R. Seitz, Thomas J. Skelly, and Richard W. Smirl as the individuals to be elected as Trustees of the Trust. Mr. Peterson, Ms. Pollina, Mr. Reaves, Mr. Seeley, Ms. Seitz, Mr. Skelly and Mr. Smirl are all current Trustees of the Trust. Mr. Peterson, Mr. Reaves, Mr. Seeley, Ms. Seitz, and Mr. Skelly were elected to the Board by shareholders in 2006. Mr. Smirl and Ms. Pollina were appointed to the Board in 2010 and 2011, respectively, by the then sitting Board of Trustees of the Trust in accordance with the provisions of the Trust’s Declaration of Trust and Section 16(a) of the Investment Company Act of 1940, as amended (the “1940 Act”). Mr. Avedisian is not currently a Trustee of the Trust and upon the recommendation of the Nominating and Governance Committee is being presented for election to fill the vacancy created when a Trustee retired from the Board on February 22, 2012. Ms. Pollina and Messrs. Peterson, Reaves, Seeley and Skelly (the “Independent Trustees”) and Mr. Avedisian (the “Nominee”) are not interested persons of the Trust as defined in the 1940 Act. Ms. Seitz and Mr. Smirl are considered interested persons of the Trust as defined in the 1940 Act (“Interested Trustees”) due to their employment with the Trust’s investment advisor and principal underwriter, William Blair & Company, L.L.C. (“William Blair” or the “Advisor”).

Each nominee elected to the Board will serve as a Trustee of the Trust until the election and qualification of a successor or until death, retirement, or resignation or removal as provided for in the Declaration of Trust. It is the Board’s policy that retirement for Independent Trustees occurs no later than at the conclusion of the first regularly scheduled Board meeting of the Trust’s fiscal year that occurs after the earlier of (a) the Trustee’s 72nd birthday or (b) the 15th anniversary of the date that the Independent Trustee was first elected or appointed as a member of the Board of Trustees.

All the nominees listed below have consented to serve as Trustees of the Trust, if elected. In case any nominee shall be unable or shall fail to act as a Trustee by virtue of an unexpected occurrence, persons named as proxies will vote in their discretion for such other nominee or nominees as the current Trustees may recommend. The following table presents certain information about the nominees. Each nominee’s year of birth is set forth after his or her name. The mailing address for each nominee is 222 West Adams Street, Chicago, Illinois 60606.

Nominees for Election as Trustees

Independent Trustees/Nominee

3

4

Interested Trustees

5

Board of Trustees

The primary responsibility of the Board of Trustees is to represent the interests of the shareholders of the Trust and to provide oversight of the management of the Trust. The slate of nominees proposed for election at the Meeting is comprised of two individuals (25% of the proposed Board) who would be Interested Trustees and six individuals (75% of the proposed Board) who would be Independent Trustees. SEC rules currently require a majority of the board members of a fund to be “independent” if the fund takes advantage of certain exemptive rules under the 1940 Act.

The Trust’s day to day operations are managed by the Advisor and other service providers who have been approved by the Board. In light of the general characteristics of the Trust, including the number of funds, the nature of the funds’ investments and the historical relationship between the Trust and the Advisor, the Board has developed a leadership structure that encourages all Trustees to participate equally in Trust governance. The Board believes that the leadership structure fosters the type of meaningful dialogue between the Advisor and the Independent Trustees that results in an appropriate balance of cooperation with and oversight of the Advisor. The Board has elected Michelle Seitz, an Interested Trustee, to serve as Chairperson of the Board and because of the Board’s leadership structure, no Independent Trustee has been designated as a lead independent trustee.

Generally, the Board acts by majority vote of all the Trustees, including a majority vote of the Independent Trustees if required by applicable law. The Board has established two standing committees, the Audit Committee and the Nominating and Governance Committee, each comprised entirely of the Independent Trustees, to which it has delegated certain responsibilities as described below. The Board and its committees meet periodically throughout the year to oversee the Trust’s activities, including reviewing at one or more meetings, the Trust’s arrangements with the Advisor and other service providers, the operation of the Trust’s investment policies, compliance and regulatory matters and the Funds’ investment performance. The Independent Trustees are represented by independent legal counsel at Board and committee meetings. As part of its general oversight of the Trust, the Board is involved in the risk oversight of the Trust directly and through its committees. The Board reviews the investment performance of the funds with the Advisor, including meeting regularly with the portfolio managers, at each of its regularly scheduled quarterly Board meetings. In addition, the Board must approve any material changes to a Fund’s investment policies or restrictions. With respect to compliance matters, the Trust’s Chief Compliance Officer provides the annual compliance report required by Rule 38a-1 under the 1940 Act, a quarterly report to the Board regarding the operation of the Trust’s compliance policies and procedures, including any material compliance issues that arose during the quarter, and meets in executive session with the Audit Committee at its quarterly meetings. With respect to valuation, the Board and its Valuation Committee oversee a pricing committee comprised of Trust officers and Advisor personnel and the Board has approved Valuation Procedures, including fair valuation procedures, applicable to valuing the Funds’ securities, which the Board reviews at least annually. The Audit Committee is responsible for monitoring the Trust’s accounting policies, financial reporting and internal control systems, as well as the work of the independent registered public accounting firm and the Audit Committee reports its activities to the Board on a regular basis. The Nominating and Governance Committee is primarily responsible for the identification and recommendation of individuals for Board membership and for overseeing the administration of the Trust’s Governance Procedures and Guidelines and the Nominating and Governance Committee reports its activities to the Board on a regular basis.

During 2011, the Board met five times. Each current Trustee attended at least 75% of the respective meetings of the Board and the Committees (if a member thereof) held during 2011.

The members of the Audit Committee, all of whom are Independent Trustees, include Messrs. Peterson (Chairperson), Reaves, Seeley and Skelly and Ms. Pollina. It is expected that, if elected, Mr. Avedisian will be appointed as a member of the Audit Committee. The Audit Committee held four meetings in 2011.

The members of the Nominating and Governance Committee, all of whom are Independent Trustees, include Messrs. Reaves (Chairperson), Peterson, Seeley and Skelly and Ms. Pollina. It is expected that, if elected,

6

Mr. Avedisian will be appointed as a member of the Nominating and Governance Committee. A copy of the Nominating and Governance Committee Charter is attached as Appendix A. The Nominating and Governance Committee held eight meetings in 2011.

Pursuant to the Trust’s Governance Procedures and Guidelines, the Nominating and Governance Committee will independently evaluate candidates for Board membership. In evaluating candidates for Board membership, the Nominating and Governance Committee will take into account such factors as it deems appropriate, including the professional experience, education and skills of the candidate and the overall diversity of the Board’s composition. The Nominating and Governance Committee believes the Board generally benefits from diversity of background, experience and views among its members, and considers this a factor in evaluating the composition of the Board, but has not adopted any specific policy in this regard. Suggestions for candidates may be submitted to the Committee by other Trustees, by shareholders or by the Advisor. Shareholders may submit suggestions for candidates by sending a resume of the candidate to the Secretary of the Trust, 222 West Adams Street, Chicago, Illinois 60606, for the attention of the Chairperson of the Nominating and Governance Committee. There is no difference in the manner in which the Nominating and Governance Committee evaluates candidates recommended by shareholders versus candidates recommended by other parties. Each Trustee/Nominee was recommended to the Board as a candidate for election by the Nominating and Governance Committee. Mr. Avedisian, who is not currently a Trustee of the Trust, was recommended to the Nominating and Governance Committee by the Advisor.

The members of the Valuation Committee include Mr. Smirl and Mr. Skelly. The other Independent Trustees are designated as alternate members in the event that Mr. Skelly is unavailable. The Valuation Committee held no meetings in 2011.

The Board has adopted a policy that absent personal circumstances that do not permit a Trustee to attend, all Trustees are expected to be present at each meeting of shareholders in order to facilitate communications with shareholders. Under circumstances where no Trustee is available to attend a shareholders meeting, the Board may designate a senior officer of the Trust to be present and to report back to the Board.

Trustee/Nominee Qualifications

The following is a brief discussion of the experiences and qualifications that led to the conclusion that each Trustee/Nominee should serve as a Trustee. Generally, the professional, business and educational experience of each Trustee/Nominee was considered in determining his or her qualifications to serve as a Trustee of the Trust. Each current Trustee’s previous record of service as a Trustee or officer of the Trust was considered and served to demonstrate his or her understanding of and commitment to the Trust. With respect to each Trustee/Nominee, the Board considered, among other factors, the following experiences and qualifications:

The Board considered Vann A. Avedisian’s professional experience serving in various executive positions with companies in the real estate industry, including co-founding and serving as a Managing Director of Oxford Capital Partners, Inc. and, currently, directing the capital market activities of Highgate Holdings, where he is a Principal. The Board considered the executive, financial, operations and risk management experience that Mr. Avedisian gained over the course of his career. The Board also considered Mr. Avedisian’s experience serving as a director of various private organizations, including service as the compensation committee chair of a large privately-owned business.

The Board considered Phillip O. Peterson’s professional training and experience as a certified public accountant and auditor, including his experience as a partner of KPMG overseeing a large group of audit, tax and consulting personnel providing professional services to mutual fund and investment management clients. The Board considered the executive, financial, audit and investment experience that Mr. Peterson gained over the course of his career. The Board also considered Mr. Peterson’s contributions to various published guides addressing generally accepted auditing and accounting standards for mutual fund companies. Further, the Board considered Mr. Peterson’s experience serving on the boards of other mutual fund complexes.

7

The Board considered Lisa A. Pollina’s professional experience serving in various executive and consulting positions with companies in the global financial services industry, including serving as a Global Financial Institutions Executive with Bank of America and currently, as a senior advisor to the head of RBC Financial Group’s International Banking and Insurance division. The Board considered the executive, financial, operations, investment and risk management experience that Ms. Pollina gained over the course of her career. The Board also considered Ms. Pollina’s experience serving as a director or trustee of various public and private organizations.

The Board considered Donald J. Reaves’s professional experience serving in various executive positions at major U.S. universities, including Chief Financial Officer at the University of Chicago and Brown University, and currently, as Chancellor of Winston-Salem State University. The Board considered the executive, financial, audit, investment and risk management experience that Mr. Reaves gained over the course of his career. The Board also considered Mr. Reaves’ experience serving as a director or trustee of various public and private organizations, including serving in multiple leadership positions on such boards.

The Board considered Donald L. Seeley’s professional experience serving as treasurer or chief financial officer of several public companies, including most recently as Chief Financial Officer of True North Communications, Inc. The Board also considered Mr. Seeley’s experience teaching a graduate portfolio management course at the University of Arizona. The Board considered the executive, financial, audit and investment experience that Mr. Seeley gained over the course of his career. The Board also considered Mr. Seeley’s experience serving as a director or trustee of various public and private organizations, including service as the audit committee chair of four public companies over the past ten years.

The Board considered Thomas J. Skelly’s professional experience serving in various executive positions at Accenture, including his experience as the managing partner of Accenture’s U.S. operations and as the chairman of the Accenture Pension Fund. The Board considered the executive, operations, information technology, financial and investment experience that Mr. Skelly gained over the course of his career. The Board also considered Mr. Skelly’s experience serving as a director or trustee of a public company and various private organizations. Further, the Board considered Mr. Skelly’s service on various advisory boards for private and public companies.

The Board considered Michelle R. Seitz’s professional experience in the financial services industry, including as a Partner of William Blair & Company, L.L.C. where she serves as the head of William Blair’s Investment Management division. The Board considered the executive, investment and financial experience that Ms. Seitz gained over the course of her career. The Board also considered that because of Ms. Seitz’s positions with William Blair, she is involved in the day-to-day management of the Advisor and the Trust.

The Board considered Richard W. Smirl’s professional training and experience as an attorney and his executive and operational experience gained as a Partner of William Blair & Company, L.L.C., including serving as Chief Operating Officer for William Blair’s Investment Management division. The Board considered the executive, legal, and operational experience that Mr. Smirl gained over the course of his career. The Board considered that because of Mr. Smirl’s positions with William Blair, he is involved in the day-to-day management of the Advisor and the Trust. The Board also noted that Mr. Smirl served as the Chief Compliance Officer for the Trust from 2004 to 2009.

References to the experience and qualifications of each Trustee/Nominee are pursuant to requirements of the Securities and Exchange Commission, do not constitute holding out of the Board or any Trustee/Nominee as having any special expertise and shall not impose any greater responsibility or liability on any such person or on the Board by reason thereof.

8

Shareholder Communication with the Trustees

To facilitate shareholder communications with the Board (or with any individual Trustee), shareholders are instructed to forward correspondence (including suggestions for Trustee candidates) by U.S. mail or other courier service to the Secretary of the Trust, 222 West Adams Street, Chicago, Illinois 60606. Correspondence addressed to the Board will be forwarded to the Chairperson of the Nominating and Governance Committee and correspondence addressed to a specific Trustee will be forwarded to that Trustee.

Compensation of Trustees

Independent Trustees receive an annual retainer of $40,000 plus $5,000 for each Board meeting attended in person plus expenses, $2,000 for each Board and/or committee meeting by telephone and $5,000 for committee meetings held on a different day from a Board meeting (except for meetings of the Valuation Committee for which the Trustees receive no additional compensation). Chairpersons of the Audit Committee and Nominating and Governance Committee each receive an additional retainer of $4,000 for serving in such positions. The Independent Trustees receive one-half of the annual retainer in cash and the other half is invested in Fund shares as directed by the Independent Trustees. The Interested Trustees and officers affiliated with the Advisor receive no compensation from the Trust.

The following table sets forth the compensation earned from the Trust for the year ended December 31, 2011 by the Independent Trustees:

Trustee Fund Ownership

Each of the current Trustees own shares of the Funds. Appendix B hereto lists the dollar range of shares of each Fund beneficially owned by each Independent Trustee/Nominee and Interested Trustee and the amount of shares each Independent Trustee/Nominee and Interested Trustee owns individually and as a group with the officers of Trust. Appendix B also lists all shareholders known to the Trust to beneficially own more than 5% of any class of any Fund as of June 1, 2012. Finally, Appendix B contains information on certain Independent Trustees’ holdings in certain affiliates of the Advisor.

Fund Officers

The following table presents information about the executive officers of the Trust. Each individual’s year of birth is set forth after his or her name. The Trust’s officers, except the Chief Compliance Officer, are elected annually by the Board. The Trust’s Chief Compliance Officer is designated by the Board and may only be removed by action of the Board, including a majority of the Independent Trustees. The Length of Time Served for all officers indicates the year the individual first became an officer of the Trust. The mailing address for each officer is 222 West Adams Street, Chicago, Illinois 60606.

9

10

11

12

Required Vote

Election of the current Trustees requires the affirmative vote of more than 50% of the shares of the Trust present at the Meeting, either in person or by proxy. Election of Mr. Avedisian to fill the current vacancy on the Board requires the affirmative vote of a plurality of the shares of the Trust entitled to vote at the Meeting.

Recommendation of the Board

The Board recommends that shareholders vote FOR the election of the each Trustee/Nominee named above.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

The Board, including the Independent Trustees, has selected Ernst & Young LLP (“E&Y”) to act as the independent registered public accounting firm to audit the books and records of the Trust for the current fiscal year. E&Y has served the Trust in this capacity since the Trust was organized and has no direct or indirect financial interest in the Trust or any Fund except as the independent registered public accounting firm. A representative of E&Y is expected to be present, in person or by telephone, at the Meeting and will be available to respond to any appropriate questions raised at the Meeting and may make a statement.

In connection with the audit of the 2011 financial statements, the Trust entered into an engagement letter with E&Y. The terms of the engagement letter required by E&Y, and agreed to by the Audit Committee, include provisions in which the parties consent to the sole jurisdiction of the courts of the State of Illinois located in Cook County or in the courts of the United States of America located in the Northern District of Illinois, as well as a waiver of right to a trial by jury and an exclusion of punitive damages.

Audit Fees

For the fiscal years ended December 31, 2010 and 2011 E&Y, billed the Trust $501,000 and $559,500, respectively, for professional services rendered for the audit of the Trust’s annual financial statements or services that are normally provided in connection with statutory and regulatory filings.

Audit-Related Fees

For the fiscal years ended December 31, 2010 and 2011, E&Y billed the Trust $3,000 and $8,000, respectively, for assurance and related services that are reasonably related to the performance of the audit of the Trust’s financial statements and that are not reported above, such as reviewing prospectuses and SEC filings. For the fiscal years ended December 31, 2010 and 2011, E&Y provided no audit-related services to William Blair or any of its control affiliates that were for engagements directly related to the Trust’s operations and financial reporting.

Tax Fees

For the fiscal years ended December 31, 2010 and 2011, E&Y billed the Trust $117,850 and $138,500, respectively, for professional services rendered for tax compliance, tax advice and tax planning. Such services

13

consisted of preparation of tax returns, year-end distribution review, qualifying dividend income analysis and computation of foreign tax credit pass through. For the fiscal years ended December 31, 2011 and 2012, E&Y did not bill William Blair or any of its control affiliates for any tax services that were for engagements directly related to the Trust’s operations and financial reporting.

All Other Fees

For the fiscal years ended December 31, 2010 and 2011, E&Y did not bill the Trust for products and services other than the services reported above. For the fiscal years ended December 31, 2010 and 2010, E&Y provided no other services to William Blair or any of its control affiliates that were for engagements directly related to the Trust’s operations and financial reporting.

Audit Committee Pre-Approval Policies and Procedures

Pursuant to Trust’s Audit Committee Charter (the “Charter”), the Audit Committee is responsible for pre-approving any engagement of the principal accountant to provide non-prohibited services to the Trust, including the fees and other compensation to be paid to the principal accountant, to the extent required by Rule 2-01(c)(7) of Regulation S-X. Pursuant to the Charter, the Audit Committee is also responsible for pre-approving any engagement of the principal accountant, including the fees and other compensation to be paid to the principal accountant, to provide non-audit services to the Trust’s investment advisor (or any entity controlling, controlled by, or under common control with the adviser that provides ongoing services to the Trust), if the engagement relates directly to the operations and financial reporting of the Trust, to the extent required by Rule 2-01(c)(7) of Regulation S-X. The Chair of the Audit Committee may grant the pre-approval referenced above for non-prohibited and non-audit services. All such delegated pre-approvals will be presented to the Audit Committee no later than the next Audit Committee meeting.

Non-Audit Fees

For the fiscal years ended December 31, 2010 and 2011, E&Y billed the Trust $117,850 and $138,500, respectively, in non-audit fees (tax services). For the same periods, E&Y billed William Blair and its control affiliates $97,000 and $119,000 (Custodian SAS70 Report), respectively, in non-audit fees. The Trust’s Audit Committee has considered whether the provision of non-audit services that were rendered to William Blair or any of its control affiliates that were not pre-approved pursuant to Rule 2-01(c)(7)(ii) of Regulation S-X is compatible with maintaining E&Y’s independence.

MISCELLANEOUS

Investment Manager and Principal Underwriter

William Blair, 222 West Adams Street, Chicago, Illinois 60606, is the Trust’s investment advisor and manager. William Blair is responsible for providing investment advisory and management services to the Funds, including making decisions regarding Fund portfolio transactions, and administers its business affairs, furnishes office facilities and equipment, provides clerical, bookkeeping and administrative services, provides shareholder and information services. William Blair is also the principal underwriter and distributor of the Trust and acts as agent of the Trust in the sale of its shares. William Blair was founded over 75 years ago by William McCormick Blair. Today, the firm has over 1,000 employees including 170 partners.

The Investment Management Department oversees the assets of the Trust, along with corporate pension plans, endowments and foundations and individual accounts. The department currently manages over $46 billion in equities, fixed-income securities and cash equivalents.

The Advisor firmly believes that clients are best served when portfolio managers are encouraged to draw on their experience and develop new ideas. This philosophy has helped build a hard-working, results-oriented team of over 46 portfolio managers, supported by a team of analysts. The Advisor is registered as an investment advisor under the Investment Advisers Act of 1940.

14

Proxy Solicitation and Expenses

In addition to solicitations by mail, solicitations also may be made by telephone, through the Internet or in person by officers or employees of the Trust and by certain financial services firms and their representatives, who will receive no extra compensation for their services. Boston Financial Data Services (“BFDS”) has been engaged to assist in the solicitation of proxies for the Trust at an estimated cost of $200,000, plus reimbursement for reasonable expenses. However, the exact cost will depend on the amount and types of services rendered.

Shareholders may vote by telephone or over the Internet. Shareholders who vote by telephone or over the Internet will have an opportunity to review their voting instructions and make any necessary changes before submitting their voting instructions and terminating their telephone call or Internet link. If a shareholder wishes to participate in the Meeting, but does not wish to give a proxy by telephone or over the Internet, the shareholder may still submit the proxy card originally sent with this proxy statement or attend the Meeting in person. Should shareholders require additional information regarding the proxy or replacement a proxy card, they may contact BFDS toll-free at 855-844-8646.

Adjournment

If a quorum is not present, the Meeting may be adjourned to a later date by the affirmative vote of the shares present, in person or by proxy, at the Meeting. In the event that a quorum is present at the Meeting, but sufficient votes to elect the Trustees or approve the amended fundamental investment restrictions are not received, the persons named as proxies may propose one or more adjournments of the Meeting to permit further solicitation of proxies. Any such adjournment will be approved by the affirmative vote of the shares present, in person or by proxy, at the Meeting. It is anticipated that the persons named as proxies would vote in favor of any adjournment. The Meeting may be adjourned without further notice to shareholders, but the Meeting may not be adjourned for more than six months beyond the originally scheduled meeting date.

Proposals of Shareholders

The Trust does not generally hold annual shareholders’ meetings, but will hold special meetings as required or deemed desirable. Because the Trust does not hold regular shareholders’ meetings, the anticipated date of the next shareholders’ meeting (if any) cannot be provided. Shareholders wishing to submit proposals for inclusion in a proxy statement for a subsequent shareholders’ meeting of the Trust should send their written proposals to the Secretary of the Trust at 222 West Adams Street, Chicago, Illinois 60606. Proposals must be received in a reasonable time before the Trust begins to print and mail its proxy materials for the meeting. The timely submission of a proposal does not guarantee its inclusion.

Other Matters to Come Before the Meeting

The Board is not aware of any matters that will be presented for action at the Meeting other than the Proposal set forth herein. Should any other matters requiring a vote of shareholders arise, the proxy in the accompanying form will confer upon the person or persons entitled to vote the shares represented by such proxy the discretionary authority to vote the shares with respect to any such other matters.

Householding Information

In order to reduce the amount of mail you receive and to help reduce Fund expenses, the Trust generally sends a single copy of any shareholder report and prospectus to each household. If you do not want the mailing of these documents to be combined with those for other members of your household, please call 1-800-742-7272.

A copy of the Trust’s annual and semi-annual report is available without charge upon request by writing to the Trust, 222 West Adams Street, Chicago, Illinois 60606 or by calling 1-800-742-7272. Reports are also available on the William Blair Funds website at www.williamblairfunds.com or at the website of the Securities and Exchange Commission at www.sec.gov.

15

Please complete, sign and return the enclosed proxy card(s) (or take advantage of available telephonic or Internet voting procedures; see the proxy card for instructions) promptly. No postage is required if mailed in the United States.

16

Appendix A

WILLIAM BLAIR FUNDS

NOMINATING AND GOVERNANCE COMMITTEE CHARTER

ADOPTED OCTOBER 24, 2000,

AMENDED OCTOBER 26, 2004,

APRIL 19, 2005, OCTOBER 25, 2005

AND OCTOBER 25, 2011

The Nominating and Governance Committee is a committee of the Board of the Trust. Its primary function is to identify and recommend individuals for membership on the Board and oversee the administration of the Board Governance Procedures and Guidelines.

The Nominating and Governance Committee shall be comprised of three or more board members as determined by the Board, each of whom shall be an independent trustee and free from any relationship that, in the opinion of the Board, would interfere with the exercise of his or her independent judgment as a member of the Nominating and Governance Committee. For purposes of the Nominating and Governance Committee, a trustee is independent if he or she is not an “interested person” of the Trust as that term is defined in the Investment Company Act of 1940.

The members of the Nominating and Governance Committee shall be elected by the Board annually and serve until their successors shall be duly elected and qualified. The Chair shall be elected by the members of the Nominating and Governance Committee by majority vote.

The Nominating and Governance Committee shall meet two times annually, or more frequently as circumstances dictate. Special meetings (including telephone meetings) may be called by the Chair or a majority of the members of the Nominating and Governance Committee upon reasonable notice to the other members of the Nominating and Governance Committee.

To fulfill its responsibilities and duties the Nominating and Governance Committee shall:

A-1

A-2

Appendix B

FUND OWNERSHIP

The following table sets forth, for each Independent Trustee/Nominee and Interested Trustee, the dollar range of shares owned in each Fund of the Trust as of April 30, 2012.

B-1

The following table sets forth, for each Independent Trustee/Nominee and Interested Trustee and for the Independent Trustees/Nominee, the Interested Trustees and the officers as a group, the amount of shares beneficially owned in each Fund as of April 30, 2012.

B-2