Toronto Montréal Calgary Ottawa Vancouver New York | | January 22, 2021 Bank of Montreal 1 First Canadian Place 18th Floor Toronto, ON M5X 1A1 Dear Sirs/Mesdames: Bank of Montreal – US$1,000,000,000 Fixed/Floating Rate Senior Notes due 2027 We have acted as Canadian counsel to Bank of Montreal (the “Bank”) in connection with the issue and sale today (the “Offering”) by the Bank of US$1,000,000,000 aggregate principal amount of the Bank’s Fixed/Floating Rate Senior Notes due 2027 (the “Notes”), constituting part of the Senior Medium-Term Notes, Series F of the Bank, pursuant to the distribution agreement dated April 20, 2020 (the “Distribution Agreement”) between, among others, the Bank and BMO Capital Markets Corp., the terms agreement dated January 19, 2021 (the “Terms Agreement”) between the Bank and BMO Capital Markets Corp., BofA Securities, Inc., Citigroup Global Markets Inc., Goldman Sachs & Co. LLC and HSBC Securities (USA) Inc., as representatives of the agents named therein, and the Senior Indenture dated as of January 25, 2010, as it may be amended and supplemented from time to time, including as supplemented by the First Supplemental Indenture thereto, dated September 23, 2018 (collectively, the “Indenture”), between the Bank and Wells Fargo Bank, National Association, as trustee. Capitalized terms used herein and not otherwise defined herein have the meanings ascribed to such terms in the Terms Agreement. We have examined copies, certified or otherwise authenticated to our satisfaction, of the following: (i) the Distribution Agreement; (ii) the Terms Agreement; (iii) the Indenture; (iv) the registration statement of the Bank on Form F-3 dated March 23, 2020 and Amendment No. 1 thereto dated April 20, 2020 (collectively, the “Registration Statement”); and (v) the prospectus of the Bank dated April 20, 2020 included in the Registration Statement (the “Basic Prospectus”) as supplemented by the prospectus supplement dated April 20, 2020 specifically relating to the Senior Medium-Term |