Exhibit 5.1

March 9, 2018   Discover Funding LLC 12 Read’s Way New Castle, Delaware 19720		Mayer Brown LLP 71 South Wacker Drive Chicago, Illinois 60606-4637   Main Tel (312) 782-0600 Main Fax (312) 701-7711 www.mayerbrown.com

Class A(2018-1) Notes, Registration Statement on Form SF-3

(No. 333-205455)

We have acted as special counsel for Discover Bank, a Delaware banking corporation (“Discover Bank”), Discover Funding LLC, a Delaware limited liability company (“Discover Funding”), and Discover Card Execution Note Trust (the “Note Issuance Trust”), in connection with the offering of the Class A(2018-1) notes (the “Notes”) described in the prospectus dated March 8, 2018 (the “Prospectus”), which has been filed with the Securities and Exchange Commission (the “Commission”) pursuant to Rule 424(b) of the Securities and Exchange Act of 1933, as amended (the “Act”). The Notes will be sold pursuant to an Underwriting Agreement, dated as of March 8, 2018 (the “Underwriting Agreement”), among Discover Bank, Discover Funding, the Note Issuance Trust and the representatives of the underwriters named in the Prospectus. The Notes will be issued pursuant to the Amended and Restated Indenture, dated as of December 22, 2015 (the “Indenture”), between the Note Issuance Trust and U.S. Bank National Association, as Indenture Trustee (the “Indenture Trustee”), as supplemented by the Second Amended and Restated Indenture Supplement, dated as of December 22, 2015 (the “Indenture Supplement”), between the Note Issuance Trust and the Indenture Trustee, and the Terms Document, to be dated on or about March 14, 2018 (the “Terms Document”), between the Note Issuance Trust and the Indenture Trustee. Unless otherwise defined herein, all capitalized terms used herein shall have the meanings assigned to them in the Indenture.

We have examined executed copies of the above captioned Registration Statement, as amended (the “Registration Statement”), registering asset-backed notes representing debt of the Note Issuance Trust, the Indenture, the Indenture Supplement, the Terms Document and such other documents as we have deemed necessary for the purposes of this opinion (collectively, the “Transaction Documents”). We are familiar with the proceedings taken by Discover Funding in connection with the authorization of the issuance and sale of the Notes, and have examined such documents and such questions of law and fact as we have deemed necessary in order to express the opinion hereinafter stated.

We are opining herein as to the effect on the subject transactions of only United States federal law, the laws of the State of New York (excluding any municipal laws), the banking laws of the State of Delaware, the limited liability company laws of the State of Delaware and the Delaware Statutory Trust Act and we express no opinion with respect to the applicability thereto or the effect thereon of the laws of any other jurisdiction or as to any matters of municipal law or the laws of any local agencies within any state.

Mayer Brown LLP operates in combination with other Mayer Brown entities (the “Mayer Brown Practices”), which have offices in North America, Europe and Asia and are associated with Tauil & Chequer Advogados, a Brazilian law partnership.

MAYER BROWN LLP

Discover Funding LLC

March 9, 2018

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We have assumed that the purchase price for the Notes will be paid to Discover Funding by the underwriters named in the Prospectus.

In rendering the opinions set forth herein, we have relied upon and assumed:

On the basis of the foregoing examination and assumptions, and upon consideration of applicable law, it is our opinion that the Notes are in proper form, and when executed, authenticated and delivered as specified in the Indenture and delivered against the payment of consideration specified in the Underwriting Agreement will be legal and binding obligations of the Note Issuance Trust, enforceable against the Note Issuance Trust in accordance with their terms.

Our opinion set forth above is subject to the effects of bankruptcy, insolvency, fraudulent conveyance, reorganization, moratorium and other similar laws relating to or affecting creditors’ rights generally and general equitable principles (whether considered in a proceeding in equity or at law) and by the discretion of the court before which any proceeding therefore may be brought.

MAYER BROWN LLP

Discover Funding LLC

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We hereby consent to the filing of this letter as part of Discover Funding’s Current Report on Form 8-K, dated of even date herewith for incorporation in the Registration Statement and to the references to this firm under the heading “Legal Matters” in the Prospectus, without admitting that we are “experts” within the meaning of the Act or the rules and regulations of the Commission issued thereunder, with respect to any part of the Registration Statement.