Exhibit 5.1

1095 Avenue of the Americas New York, NY 10036-6797 +1  212  698  3500  Main +1  212  698  3599  Fax www.dechert.com

August 18, 2020

Griffon Corporation

712 Fifth Avenue, 18^th Floor

New York, New York 10019

Ladies and Gentlemen:

We have acted as special counsel to Griffon Corporation, a Delaware corporation (the ”Company”), in connection with the preparation of a Registration Statement on Form S-3 (Registration No. 333-224727) (the “Registration Statement”) and the related prospectus contained therein (the “Base Prospectus”), which was filed by the Company with the Securities and Exchange Commission (the “Commission”) and became immediately effective upon filing on May 7, 2018 pursuant to Rule 462(e) under the Securities Act of 1933, as amended (the “Act”), as supplemented by the prospectus supplement, dated August 11, 2020 (the “Preliminary Prospectus Supplement”) and the final prospectus supplement filed with the Commission on August 17, 2020 (the “Prospectus Supplement” and, together with the Base Prospectus and the Preliminary Prospectus, the “Prospectus”) in connection with (i) the sale by the Company of 8,000,000 shares (the ”Company Securities”) of the Company’s common stock, par value $0.25 per share (the “Common Stock”), (ii) the offering by the Company of up to 700,000 shares (the “Company Option Securities”) of Common Stock pursuant to an overallotment option (the “Overallotment Option”) provided in the Underwriting Agreement (as defined below) and (iii) the offering by Ronald J. Kramer (the “Selling Stockholder”) of up to 500,000 shares of Common Stock (the “Selling Stockholder Securities” and, together with the Company Securities and the Company Option Securities, the “Offered Securities”) pursuant to the Overallotment Option. All of the Offered Securities are to be sold by the Company or the Selling Stockholder, as applicable, as described in the Registration Statement and the related Prospectus.

In connection with this opinion (this “Opinion”), we have examined originals or copies (in each case signed, certified or otherwise proven to our satisfaction to be genuine) of: (i) the Registration Statement and the Prospectus; (ii) the Company’s Restated Certificate of Incorporation (as amended to date) (the “Certificate of Incorporation”); (iii) the Company’s Amended and Restated Bylaws as currently in effect (the ”Bylaws”); (iv) the Underwriting Agreement (the “Underwriting Agreement”), dated August 13, 2020, for which Robert W. Baird & Co. Incorporated is acting as Representative (as defined in the Underwriting Agreement) to the several underwriters named therein; (v) minutes evidencing corporate actions of the Company authorizing, among other things,

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the offering and sale of the Offered Securities; and (vi) a certificate of an officer of the Company as to matters of fact material to this Opinion.

In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as original documents, and the conformity to original documents of all documents submitted to us as copies, the legal capacity of natural persons who are signatories to the documents examined by us and the legal power and authority of all persons signing on behalf of parties (other than the Company) to all documents.

We have further assumed that the Offered Securities will be sold in the manner stated in the Registration Statement and the Prospectus, and in compliance with the applicable provisions of the Act and the rules and regulations of the Commission thereunder and the securities or “Blue Sky” laws of various states and the terms and conditions of the Underwriting Agreement.

Our opinions set forth herein are based solely upon the laws of the State of New York and the General Corporation Law of the State of Delaware as in effect on the date hereof, and we express no opinion with respect to any other laws, rules or regulations (including, without limitation, the application of the securities or “Blue Sky” laws of any state to the offer and/or sale of the Offered Securities).

Based upon and subject to the foregoing, we are of the opinion that, (i) as of the date hereof, the Company Securities have been duly authorized, validly issued, fully paid and non-assessable, (ii) the Company Option Securities will be duly authorized, validly issued, fully paid and non-assessable when sold by the Company in accordance with the Registration Statement and the Prospectus, and (iii) the Selling Stockholder Securities will be duly authorized, validly issued, fully paid and non-assessable when sold by the Selling Stockholder in accordance with the Registration Statement and the Prospectus.

We hereby consent to the filing of this Opinion as an exhibit to the Company’s Current Report on Form 8-K filed with the Commission on August 18, 2020 and as an exhibit to any application under the securities or other laws of any state of the United States which relate to the offer and sale of the Offered Securities. We further consent to the use of our name under the heading “Legal Matters” in the Prospectus. In giving such consent, we do not admit hereby that we come within the category of persons whose consent is required under Section 7 of the Act, or the rules and regulations of the Commission thereunder.

This Opinion is furnished to you in connection with the offer and sale of the Offered Securities and is not to be used, circulated, quoted or otherwise relied upon for any other purpose, except as expressly provided in the preceding paragraph. This Opinion is furnished as of the date hereof and we disclaim any undertaking to update this Opinion aVfter the date hereof or to advise you of any

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subsequent changes of the facts stated or assumed herein or of any subsequent changes in applicable law.