Exhibit 5.1

November 7, 2019

U.S. Well Services, Inc.

1360 Post Oak Boulevard, Suite 1800

Houston, Texas 77056

Re: U.S. Well Services, Inc.

Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as counsel to U.S. Well Services, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing with the U.S. Securities and Exchange Commission (the “Commission”) of a Registration Statement on Form S-3 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration under the Securities Act and the proposed offering and sale from time to time pursuant to Rule 415 under the Securities Act by the selling stockholders named therein, together or separately, of up to 12,000,006 shares (the “Shares”) of the Company’s Class A common stock, par value $0.0001 per share (the “Common Stock”), issuable upon conversion of shares of Series A Redeemable Convertible Preferred Stock of the Company (the “Preferred Stock”) or upon the exercise of warrants of the Company (the “Warrants” and, together with the Preferred Stock, the “Convertible Securities”), in each case that were issued pursuant to the Purchase Agreement, dated as May 23, 2019, by and among the Company and the purchasers party thereto.

In arriving at the opinion expressed below, we have examined originals, or copies certified or otherwise identified to our satisfaction as being true and complete copies of the originals, of such documents, corporate records, certificates of officers of the Company and of public officials and other instruments as we have deemed necessary or advisable to enable us to render these opinions. In our examination, we have assumed the genuineness of all signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals and the conformity to original documents of all documents submitted to us as copies. As to any facts material to these opinions, we have relied to the extent we deemed appropriate and without independent investigation upon statements and representations of officers and other representatives of the Company and others.

Based on the foregoing and in reliance thereon, and subject to the assumptions, exceptions, qualifications and limitations set forth herein, we are of the opinion that when the Shares have been duly delivered upon conversion or exercise of any Convertible Security, in accordance with the terms of such Convertible Security or the instrument governing such Convertible Security providing for such conversion or exercise, and for any additional

U.S. Well Services, Inc.

November 7, 2019

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consideration specified therein, which consideration (including any consideration paid for such Convertible Security), on a per-share basis, shall in either event not be less than the par value of the Common Stock, such Shares will be validly issued, fully paid and non-assessable.

We render no opinion herein as to matters involving the laws of any jurisdiction other than the United States of America and the Delaware General Corporation Law (the “DGCL”). We are not admitted to practice in the State of Delaware; however, we are generally familiar with the DGCL as currently in effect and have made such inquiries as we consider necessary to render the opinion above. This opinion is limited to the effect of the current state of the laws of the United States of America and, to the limited extent set forth above, the laws of the State of Delaware and the facts as they currently exist. We assume no obligation to revise or supplement this opinion in the event of future changes in such laws or the interpretations thereof or such facts.

We consent to the filing of this opinion as an exhibit to the Registration Statement, and we further consent to the use of our name under the caption “Legal Matters” in the Registration Statement and the prospectus that forms a part thereof. In giving these consents, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Gibson, Dunn & Crutcher LLP