SUBSCRIPTION AGENT AGREEMENT

Exhibit 4.2

SUBSCRIPTION AGENT AGREEMENT

February 11, 2008

Registrar and Transfer Company

10 Commerce Drive

Cranford, New Jersey 07016

Ladies and Gentlemen:

In connection with your appointment as Subscription Agent in the transaction described herein, Security Bank Corporation (the “ Company ”), hereby confirms its arrangements with you as follows:

1. Rights Offering .

The Company is distributing, at no charge, to the record holders of shares of its common stock, par value $1.00 per share (“ Common Stock ”), as of 5:00 p.m., local time, on February 11, 2008 (the “ Record Date ”), non-transferable subscription rights (the “ Rights ”) to subscribe for and purchase shares of Common Stock (the “ Rights Offering ”). Each record holder is entitled to receive one Right for each share of Common Stock held by such record holder at the Record Date. Each Right entitles the holder thereof to subscribe for 0.281212 shares of Common Stock (the “ Basic Subscription Privilege ”) at a subscription price of $6.58 per full share of Common Stock (the “ Subscription Price ”). Each holder of Rights who exercises in full its Basic Subscription Privilege will be entitled, subject to certain limitations and subject to allotment, to subscribe for additional shares of Common Stock up to 100% of such holder’s pro rata share of the Unsubscribed Shares (as defined below) at the Subscription Price pursuant to the Basic Subscription Privilege (the “ Over-Subscription Privilege ”). To the extent that sufficient Unsubscribed Shares are not available to honor all Over-Subscription Privilege requests, the Unsubscribed Shares will be allocated as described in the Prospectus (as defined herein). Except as set forth below, Rights shall cease to be exercisable at 5:00 p.m., local time, on March 10, 2008, unless the Company extends the period for exercising Rights for a period not to exceed 15 days (as it may be extended, the “ Expiration Time ”). The Company shall notify you orally and confirms in writing an extension of the Expiration Time. Rights are evidenced by non-transferable rights certificates in registered form (“ Rights Certificates ”). The Rights Offering will be conducted in the manner and upon the terms and conditions set forth in the Company’s final Prospectus Supplement, dated as of February 12, 2008 (the “ Prospectus ”), which is incorporated herein by reference and made a part hereof as if set forth in full herein.

2. Appointment of Subscription Agent .

You are hereby appointed as Subscription Agent to effect the Rights Offering in accordance with the Prospectus. Each reference to you in this letter is to you in your capacity as Subscription Agent unless the context indicates otherwise.

3. Delivery of Documents .

Enclosed herewith are the following, the receipt of which you acknowledge by your execution hereof:

(a) a copy of the Prospectus, attached hereto as Annex A ;

(b) the form of Rights Certificate (with instructions), attached hereto as Annex B ;

(c) Form of Instructions for Use of Security Bank Corporation Subscription Rights, attached hereto as Annex C ;

(d) Form of Notice of Guaranteed Delivery, attached hereto as Annex D ;

(e) Form of Letter to Shareholders that are Record Holders, attached hereto as Annex E ;

(f) Form of Letter to Shareholders who are Beneficial Holders, attached hereto as Annex F ;

(g) Form of Letter to Clients of Shareholders who are Beneficial Holders, attached hereto as Annex G;

(h) Form of Nominee Holder Certification Form, attached hereto as Annex H ;

(i) Form of Beneficial Owner Election Form, attached hereto as Annex I ; and

(j) Substitute Form W-9 for use with the Rights Offering, attached hereto as Annex J (documents (c) through (j), the “ Subscription Documents ”).

As soon as is reasonably practical, you shall mail or cause to be mailed to each holder of Common Stock whose address of record is within the United States at the Record Date: (i) a Prospectus, (ii) a Rights Certificate evidencing the Rights to which such holder is entitled, (iii) the Subscription Documents and (iv) a return envelope addressed to you. Prior to your mailing of the documents, the Company will provide you with blank Rights Certificates which you will prepare and issue in the names of holders of Common Stock of record at the close of business on the Record Date and for the number of Rights to which they are entitled. The Company will also provide you with a sufficient number of copies of each of the documents to be mailed with the Rights Certificates.

You shall refrain from mailing the Prospectus, Rights Certificates and Subscription Documents to any record holder of Common Stock on the Record Date whose address of record is outside the United States or is an A.P.O. or F.P.O. address, and shall hold such Rights Certificates for the account of such shareholder subject to such shareholder making arrangements with the Subscription Agent satisfactory to you and to the Company for the exercise thereby, and following the instructions of the shareholder for the exercise of such Rights if such instructions are confirmed by counsel to the Company and are received at or before 11:00 a.m., local time, on March 10, 2008.

4. Subscription Procedure .

(a) Upon your receipt prior to the Expiration Time (by mail or delivery), as Subscription Agent, of (i) any Rights Certificate completed and endorsed for exercise, as provided on the reverse side of the Rights Certificate (except as provided below) or Nominee Holder Certification from The Depository Trust Company or other nominee of the subscriber and (ii) payment in full of the Subscription Price in U.S. funds by check, bank draft, money order payable at par (without deduction for bank service charges or otherwise) or wire transfer of immediately available funds to the order of Registrar and Transfer Company, you shall, as soon as practicable after the Expiration Time, mail to the subscriber’s registered address on the books of the Company or to the subscriber’s nominee, as applicable, certificates representing the shares of Common Stock duly subscribed for and furnish a list of all such information to the Company.

(b) As soon as reasonably practical following the Expiration Time, you shall calculate the number of shares of Common Stock to which each subscriber is entitled pursuant to such subscriber’s Over-Subscription Privilege. The Over-Subscription Privilege may only be exercised by holders who fully exercise their Basic Subscription Privilege. The maximum amount of shares of Common Stock available pursuant to the Over-Subscription Privilege shall equal the number of shares of Common Stock that have not been subscribed and paid for pursuant to the Over-Subscription Privilege (the “ Unsubscribed Shares ”), it being understood that Jonathan W. Been and Benjamin W. Griffith, III are required to exercise their Basic Subscription Privilege pursuant to Standby Purchase Agreements entered into with the Company. If there are sufficient Unsubscribed Shares at the Expiration Time to satisfy all subscriptions of record holders exercising their Rights under the Over-Subscription Privilege, each holder shall be allotted the number of Unsubscribed Shares, at the Subscription Price, having a value equal to the amount subscribed for. If there is an insufficient number of Unsubscribed Shares at the Expiration Time after record holders have exercised Rights under their Basic Subscription Privilege to satisfy all subscriptions of record holders

- 2 -

exercising their Rights under the Over-Subscription Privilege, you will allocate the Unsubscribed Shares among the record holders who exercised their Over-Subscription Privilege as follows:

(i) To the extent the aggregate Subscription Price of the maximum number of shares of Common Stock available to a record holder pursuant to its Over-Subscription Privilege is less than the amount that such record holder actually paid in connection with the exercise of its Over-Subscription Privilege, such record holder will be allocated a number of Unsubscribed Shares equal to the product of (A) the total number of Unsubscribed Shares, multiplied by (B) a fraction, the dominator of which is (x) the number of Rights distributed to such record holder and the denominator of which is (y) 18,915,078.

(ii) To the extent the amount that a record holder actually paid in connection with the exercise of its Over-Subscription Privilege is less than the aggregate Subscription Price of the maximum number of shares of Common Stock available to such record holder pursuant to its Over-Subscription Privilege, such record holder will be allocated the full number of Unsubscribed Shares for which the subscriber paid.

Any fractional shares of Common Stock to which persons exercising their Over-Subscription Privilege would otherwise be entitled pursuant to such allocation shall