Document:

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                                                                     Exhibit 4.1

                                                                          PAGE 1

                                    DELAWARE

                                 THE FIRST STATE

     I, HARRIET SMITH WINDSOR, SECRETARY OF STATE OF THE STATE OF DELAWARE, DO
HEREBY CERTIFY THE ATTACHED IS A TRUE AND CORRECT COPY OF THE CERTIFICATE OF
STATUTORY TRUST REGISTRATION OF "AMG CAPITAL TRUST I", FILED IN THIS OFFICE ON
THE TWENTY-EIGHTH DAY OF MARCH, A.D. 2006, AT 5:30 O'CLOCK P.M.

                                        /s/ Harriet Smith Windsor
                                        ----------------------------------------
                 [SEAL]                 Harriet Smith Windsor,
                                        Secretary of State

                                        AUTHENTICATION: 4630672

                                        DATE: 03-29-06

4133367   8100

060299258

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                                                         STATE OF DELAWARE
                                                        SECRETARY OF STATE
                                                     DIVISION OF CORPORATIONS
                                                   DELIVERED 05:39 PM 03/28/2006
                                                     FILED 05:30 PM 03/28/2006
                                                   SRV 060294801 - 4133367 FILE

                              CERTIFICATE OF TRUST

                                       OF

                               AMG CAPITAL TRUST I

     THIS Certificate of Trust of AMG Capital Trust I (the "Trust") is being
duly executed and filed by the undersigned to form a statutory trust under the
Delaware Statutory Trust Act (12 DEL. C. Section 3801 ET SEQ.) (the "Act").

     1. NAME. The name of the statutory trust formed hereby is AMG Capital
Trust I.

     2. DELAWARE TRUSTEE. The name and business address of the trustee of the
Trust in the State of Delaware are Christiana Bank & Trust Company, 1314 King
Street, Wilmington, DE 19801.

     3. EFFECTIVE DATE. This Certificate of Trust shall be effective upon
filing.

     IN WITNESS WHEREOF, the undersigned has executed this Certificate of Trust
in accordance with Section 381l(a) of the Act.

                                        CHRISTIANA BANK & TRUST COMPANY, not in
                                        its individual capacity but solely as
                                        Delaware Trustee

                                        By: /s/ James M. Young
                                            ------------------------------------
                                            Name: JAMES M. YOUNG
                                            Title: ASSISTANT VICE PRESIDENT

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                                                                          PAGE 1

                                    DELAWARE

                                 THE FIRST STATE

     I, HARRIET SMITH WINDSOR, SECRETARY OF STATE OF THE STATE OF DELAWARE, DO
HEREBY CERTIFY "AMG CAPITAL TRUST I" IS DULY FORMED UNDER THE LAWS OF THE STATE
OF DELAWARE AND IS IN GOOD STANDING AND HAS A LEGAL EXISTENCE SO FAR AS THE
RECORDS OF THIS OFFICE SHOW, AS OF THE TWENTY-NINTH DAY OF MARCH, A.D. 2006.

                                        /s/ Harriet Smith Windsor
                                        ----------------------------------------
                 [SEAL]                 Harriet Smith Windsor,
                                        Secretary of State

                                        AUTHENTICATION: 4630675

                                        DATE: 03-29-06

4133367   8300

060299262<Page>

                                                                     EXHIBIT 4.2

                              DECLARATION OF TRUST

DECLARATION OF TRUST, dated as of March 28, 2006, between Affiliated Managers
Group, Inc., a Delaware corporation, as sponsor (the "Sponsor"), and Christiana
Bank & Trust Company, a Delaware banking corporation, as trustee (the "Delaware
Trustee" and, together with such other trustees of the Trust as are appointed
from time to time, the "Trustees"). The Sponsor and the Trustees hereby agree as
follows:

          1. The trust created hereby shall be known as "AMG Capital Trust I" in
which name the Trustees, or the Sponsor to the extent provided herein, may
conduct the business of the Trust, make and execute contracts, and sue and be
sued.

          2. The Sponsor hereby assigns, transfers, conveys and sets over to the
Trust the sum of $1. The Trustees hereby acknowledge receipt of such amount in
trust from the Sponsor, which amount shall constitute the initial trust estate.
The Trustees hereby declare that they will hold the trust estate in trust for
the Sponsor. It is the intention of the parties hereto that the Trust created
hereby constitute a statutory trust under Chapter 38 of Title 12 of the Delaware
Code, 12 Del. Code Section 3801 ET SEQ. (the "Statutory Trust Act"), and that
this document constitute the governing instrument of the Trust. The Delaware
Trustee is hereby authorized and directed to execute and file a certificate of
trust in the office of the Secretary of State of the State of Delaware in the
form attached hereto as Exhibit A. The Trust is hereby established by the
Sponsor and the Trustees for the purpose of (i) issuing trust convertible
preferred securities ("Convertible Preferred Securities"), representing
undivided beneficial interests in the assets of the Trust in exchange for cash
and investing the proceeds thereof in convertible debentures of the Sponsor,
(ii) issuing and selling common securities ("Common Securities" and, together
with the Convertible Preferred Securities, "Trust Securities"), representing
undivided beneficial interests in the assets of the Trust to the Sponsor in
exchange for cash and investing the proceeds thereof in additional convertible
debentures of the Sponsor and (iii) engaging in such other activities as are
necessary, convenient or incidental thereto.

          3. Concurrent with the first issuance of any Trust Securities by the
Trust, the Sponsor and the Trustees intend to enter into an amended and restated
Declaration of Trust, satisfactory to each such party, to provide for the
contemplated operation of the Trust created hereby and the issuance of the
Convertible Preferred Securities and the Common Securities referred to therein.
Prior to the execution and delivery of such amended and restated Declaration of
Trust, the Trustees shall not have any duty or obligation hereunder or with
respect to the trust estate.

          4. The Sponsor and the Trustees hereby authorize and direct the
Sponsor, as the agent of the Trust, (i) to prepare an offering memorandum (the
"Offering Memorandum") in preliminary and final form in relation to the offering
and sale of the Convertible Preferred Securities to qualified institutional
buyers in reliance on Rule 144A under the Securities Act of 1933, as amended
(the "Securities Act"); (ii) to file with the Private Offering, Resales and
Trading through Automatic Linkages (PORTAL) Market ("PORTAL") and execute on
behalf of the Trust a listing application or applications and all other
applications, statements, certificates, agreements and other instruments as
shall be necessary or desirable to cause the Convertible

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Preferred Securities to be listed on PORTAL; (iii) to prepare, execute and file,
in each case on behalf of the Trust, such applications, reports, surety bonds,
irrevocable consents, appointments of attorney for service of process and other
papers and documents as shall be necessary or desirable to register the
Convertible Preferred Securities under the securities or "blue sky" laws of such
jurisdictions as the Sponsor, on behalf of the Trust, may deem necessary or
desirable; (iv) to negotiate the terms of, and execute and deliver on behalf of
the Trust, a purchase agreement among the Trust, the Sponsor and any
underwriter, dealer or agent relating to the offer and sale of the Convertible
Preferred Securities, satisfactory to each such party; and (v) to execute on
behalf of the Trust any and all documents, papers and instruments as may be
desirable in connection with any of the foregoing. In the event that any filing
referred to in any of clauses (ii) and (iii) above is required by the rules and
regulations of the applicable governmental agency, self-regulatory organization
or other person or organization or state securities or blue sky laws, to be
executed on behalf of the Trust by the Trustees, Trustees are hereby authorized
and directed to join in any such filing and to execute on behalf of the Trust
any and all of the foregoing, it being understood that no Trustee shall be
required to join in any such filing or execute on behalf of the Trust any such
document unless required by the any such law, rule or regulation and unless such
Trustee has received an indemnity satisfactory to it.

          5. This Declaration of Trust may be executed in one or more
counterparts.

          6. The number of Trustees initially shall be one (1) and thereafter
the number of Trustees shall be such number as shall be fixed from time to time
by a written instrument signed by the Sponsor which may increase or decrease the
number of Trustees; PROVIDED, HOWEVER, that to the extent required by the
Statutory Trust Act, one Trustee shall either be a natural person who is a
resident of the State of Delaware or, if not a natural person, an entity which
has its principal place of business in the State of Delaware and meets any other
requirements imposed by applicable law. Subject to the foregoing, the Sponsor is
entitled to appoint or remove without cause any Trustee at any time. Any Trustee
may resign upon 30 days prior notice to the Sponsor; provided, however, that
such notice shall not be required if it is waived by Sponsor.

          7. The recitals contained in this Declaration of Trust shall be taken
as statements of the Sponsor, and the Trustees do not assume any responsibility
for their correctness. The Trustees make no representations as to the value or
condition of the property of the Trust or any part thereof. The Trustees make no
representations as to the validity or sufficiency of this Declaration of Trust.

          8. Christiana Bank & Trust Company, in its capacity as Trustee, shall
not have any of the powers or duties of the Trustees set forth herein (except as
may be required under the Statutory Trust Act) and shall be a Trustee of the
Trust for the sole purpose of satisfying the requirements of Section 3807 of the
Statutory Trust Act and to accept the contribution referenced in Section 2.

          9. The Trust may terminate without issuing any Trust Securities at the
election of the Sponsor.

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          10. This Declaration of Trust and the rights of the parties hereunder
shall be governed by and interpreted in accordance with the laws of the State of
Delaware and all rights and remedies shall be governed by such laws without
regard to the principles of conflict of laws.

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               IN WITNESS WHEREOF, the parties hereto have caused this
Declaration of Trust to be duly executed as of the day and year first above
written.

                                        AFFILIATED MANAGERS GROUP, INC.,
                                        as Sponsor

                                        By: /s/ John Kingston, III
                                            ------------------------------------
                                        Name: John Kingston, III
                                        Title: Senior Vice President, General
                                               Counsel and Secretary

                                        CHRISTIANA BANK & TRUST COMPANY, as
                                        Delaware Trustee

                                        By: /s/ James M. Young
                                            ------------------------------------
                                        Name: James M. Young
                                        Title: Assistant Vice President

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                                    EXHIBIT A

                              CERTIFICATE OF TRUST

                                       OF

                               AMG CAPITAL TRUST I

     THIS Certificate of Trust of AMG Capital Trust I (the "Trust") is being
duly executed and filed by the undersigned to form a statutory trust under the
Delaware Statutory Trust Act (12 DEL. C. Section 3801 ET SEQ.)(the "Act").

     1. NAME. The name of the statutory trust formed hereby is AMG Capital
Trust I.

     2. DELAWARE TRUSTEE. The name and business address of the trustee of the
Trust in the State of Delaware are Christiana Bank & Trust Company, 1314 King
Street, Wilmington, DE 19801.

     3. EFFECTIVE DATE. This Certificate of Trust shall be effective upon
filing.

     IN WITNESS WHEREOF, the undersigned has executed this Certificate of Trust
in accordance with Section 3811(a) of the Act.

                                        CHRISTIANA BANK & TRUST COMPANY, not in
                                        its individual capacity but solely as
                                        Delaware Trustee

                                        By: /s/ James M. Young
                                            ------------------------------------
                                        Name: James M. Young
                                        Title: Assistant Vice President

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