Exhibit 5.1

August 26, 2008

Mannatech, Incorporated

600 S. Royal Lane, Suite 200

Coppell, Texas 75019

Re: Mannatech, Incorporated

Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as counsel to Mannatech, Incorporated, a Texas corporation (the “Company”), in connection with the preparation and filing by the Company with the Securities and Exchange Commission of a Registration Statement on Form S-8 (the “Registration Statement”), under the Securities Act of 1933, as amended (the “Act”). The Registration Statement relates to the proposed issuance by the Company of up to 1,000,000 shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share (“Common Stock”), authorized for issuance pursuant to the Mannatech, Incorporated 2008 Stock Incentive Plan, as such plan is described in the Registration Statement (the “Plan”). This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Act.

We have examined originals or certified copies of such corporate records of the Company and other certificates and documents of officials of the Company, public officials and others as we have deemed appropriate for purposes of this letter. We have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals, and the conformity to authentic original documents of all copies submitted to us as conformed, certified or reproduced copies. We have also assumed that (i) the certificates for the Shares will conform to the specimen thereof filed as an exhibit to the Registration Statement and upon issuance will have been duly countersigned by the transfer agent and duly registered by the registrar for the Common Stock or, if uncertificated, valid book-entry notations for the issuance of the Shares in uncertificated form will have been duly made in the share register of the Company; (ii) each award agreement setting forth the terms of each award granted pursuant to the Plan is consistent with the Plan and has been duly authorized and validly executed and delivered by the parties thereto; (iii) at the time of each issuance of Shares, there will be sufficient shares of Common Stock authorized for issuance under the Company’s articles of incorporation that have not otherwise been issued or reserved or committed for issuance; and (iv) the price per share paid for Shares issued pursuant to the Plan is not less than the par value of the Shares. As to various questions of fact relevant to this letter, we have relied, without independent investigation, upon certificates of public officials and certificates of officers of the Company, all of which we assume to be true, correct and complete.

1700 Pacific Avenue, Suite 4100 / Dallas, Texas 75201-4675 / 214.969.2800 / fax: 214.969.4343 / akingump.com

Page 2

August 26, 2008

Mannatech, Incorporated

Based upon the foregoing, and subject to the assumptions, exceptions, qualifications and limitations stated herein, we are of the opinion that when the Registration Statement has become effective under the Act and when the Shares have been issued, sold and delivered in compliance with the terms of the Plan and applicable federal and state securities laws and in the manner described in the Registration Statement, the Shares will be duly authorized, validly issued, fully paid and non-assessable.

The opinions and other matters in this letter are qualified in their entirety and subject to the following:

A. We express no opinion as to the laws of any jurisdiction other than the laws of the State of Texas. As used herein, the term “laws of the State of Texas” includes the statutory provisions contained in the Texas Business Corporation Act and all applicable provisions of the Texas Constitution and reported judicial decisions interpreting these laws.

B. This opinion letter is limited to the matters expressly stated herein and no opinion is to be inferred or implied beyond the opinion expressly set forth herein. We undertake no, and hereby disclaim any, obligation to make any inquiry after the date hereof or to advise you of any changes in any matter set forth herein, whether based on a change in the law, a change in any fact relating to the Company or any other person or any other circumstance.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Act and the rules and regulations thereunder.

Very truly yours,

/s/ Akin, Gump, Strauss, Hauer & Feld, L.L.P.

AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.