MIRANT SERVICES SUPPLEMENTAL BENEFIT (SAVINGS) PLAN MIRANT SERVICES SUPPLEMENTAL BENEFIT (SAVINGS) PLAN

Exhibit 10.21

MIRANT SERVICES

SUPPLEMENTAL BENEFIT (SAVINGS) PLAN

MIRANT SERVICES

SUPPLEMENTAL BENEFIT (SAVINGS) PLAN

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MIRANT SERVICES

SUPPLEMENTAL BENEFIT (SAVINGS) PLAN

ARTICLE I

PURPOSE AND ADOPTION OF PLAN

1.1 Adoption : The Mirant Services Supplemental Benefit Plan is hereby amended and restated effective as of January 1, 2009 as two separate plans — The Mirant Services Supplemental Benefit (Pension) Plan and the Mirant Services Supplemental Benefit (Savings) Plan. This Plan is a continuation of the defined contribution portion of the Mirant Services Supplemental Benefit Plan which was originally adopted effective January 1, 2001. The Mirant Services Supplemental Benefit (Pension) Plan is a continuation of the defined benefit portion of such plan.

1.2 Purpose : The Plan is designed to provide certain retirement and other deferred compensation benefits primarily for a select group of management or highly compensated employees which (1) are not otherwise payable or cannot otherwise be provided under the Savings Plan as a result of the limitations set forth under Sections 401(a)(17) or 415 of the Code and (2) compensate for lost benefits resulting from participation in the Deferred Compensation Plans. The Plan shall be an unfunded deferred compensation arrangement whose benefits shall be paid solely from the general assets of the Employing Companies.

1.3 Transferred Accounts .

(a) Funds representing the amounts in each Participant’s “Supplemental Benefit Account” (as such term is defined in the Southern Company Energy Marketing Executive Deferred Compensation and Supplemental Benefit Plan (the “SCEM Plan”)) under the SCEM Plan, as determined on the Effective Date of this Plan, were transferred to the Company’s general assets and are recorded as a bookkeeping account for such Participant under the Plan (the “SCEM Accounts”). No further contributions shall be made to the SCEM Accounts, which shall receive interest at the prime rate as published in the Wall Street Journal , compounded daily. Payments shall be made from the SCEM Accounts at the same time and in the same manner as a Participant’s Account under the Plan.

(b) Funds representing the amounts in each Employee’s “Account” (as such term is defined in the Southern Company Supplemental Benefit Plan (the “Southern Plan”)) under the Southern Plan, as determined on the Effective Date of the Plan, were transferred to the Company’s general assets. Such amounts are included in the respective Participant’s Account under the Plan and payable at the same time and in the same manner as the Participant’s Account under the Plan. With respect to an Employee whose Account under the Southern Plan was transferred to the Plan (the “Southern Account”), but who is not otherwise a Participant in the Plan, payment of the amounts in such Southern Account shall be made at the same time and in the same manner as if such Southern Account were an Account under the Plan.

ARTICLE II

DEFINITIONS

2.1 “ Account ” shall mean the total amount credited to the bookkeeping account of a Participant established by the Employing Company to reflect the interest of a Participant in the Plan resulting from a Participant’s Savings Benefit calculated in accordance with Section 5.1 and Section 1.3.

2.2 “ Beneficiary ” shall mean any person, estate, trust, or organization entitled to receive any payment under the Plan upon the death of a Participant.

2.3 “ Board ” or “Board of Managers” shall mean the Board of Managers of the Company.

2.4 “ Code ” shall mean the Internal Revenue Code of 1986, as amended from time to time.

2.5 “ Committee ” shall mean the committee as designated by the Board to be responsible for administering the Plan, and as referred to in Section 3.1 hereof.

2.6 “ Company ” shall mean Mirant Services, LLC, its successors and assigns.

2.7 “ Deferred Compensation Plans ” shall mean the Mirant Corporation Deferred Compensation Plan for Directors and Select Employees and the Mirant Corporation Deferred Compensation Plan, as amended from time to time, and any successor to such plans.

2.8 “ Effective Date ” of this Plan shall mean January 1, 2009. The Mirant Services Supplemental Benefit Plan, as the predecessor to this Plan, was originally effective January 1, 2001.

2.9 “ Employee ” shall mean an employee of an Employing Company, not covered by a collective bargaining agreement between the Employing Company and a union or other employee representative.

2.10 “ Employing Company ” shall mean the Company, any United States subsidiary or affiliate of Mirant, or any other subsidiary or affiliate of Mirant, which the Board may from time to time determine to bring under the Plan and which shall adopt the Plan, and any successor of any of them.

2.11 “ Mirant ” shall mean Mirant Corporation (f/k/a Southern Energy, Inc.), its successors and assigns.

2.12 “ Participant ” shall mean an Employee or former Employee who is eligible and participates in the Plan pursuant to Sections 4.1 and 4.2.

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2.13 “ Plan ” shall mean the Mirant Services Supplemental Benefit (Savings) Plan, as amended from time to time.

2.14 “ Plan Year ” shall mean the calendar year.

2.15 “ Savings Benefit ” shall mean the benefit described in Section 5.1.

2.16 “ Savings Plan ” shall mean the Mirant Services Employee Savings Plan, as amended from time to time.

2.17 “ Trust ” shall mean the Mirant Corporation Benefit Security Trust Agreement (adopted effective April 2, 2007).

Where the context requires, the definitions of all terms set forth in the Savings Plan and the Deferred Compensation Plans shall apply with equal force and effect for purposes of interpretation and administration of the Plan, unless said terms are otherwise specifically defined in the Plan. The masculine pronoun shall be construed to include the feminine pronoun and the singular shall include the plural, where the context so requires.

ARTICLE III

ADMINISTRATION OF PLAN

3.1 Administrator . The general administration of the Plan shall be placed in the Committee. The Committee shall consist of the members of the Company’s Mirant Benefits Committee. Any member may resign or may be removed by the Board of Managers and new members may be appointed by the Board of Managers at such time or times as the Board of Managers in its discretion shall determine. The Committee shall be chaired by the Chairman of the Mirant Benefits Committee and may select a Secretary (who may, but need not, be a member of the Committee) to keep its records or to assist it in the discharge of its duties. A majority of the members of the Committee shall constitute a quorum for the transaction of business at any meeting. Any determination or action of the Committee may be made or taken by a majority of the members present at any meeting thereof, or without a meeting by resolution or written memorandum concurred in by a majority of the members.

3.2 Powers . The Committee shall administer the Plan in accordance with its terms and shall have all powers necessary to carry out the provisions of the Plan more particularly set forth herein. It shall have the discretion to interpret the Plan and shall determine all questions arising in the administration, interpretation and application of the Plan. Any such determination by it shall be conclusive and binding on all persons. It may adopt such regulations as it deems desirable for the conduct of its af