Maynard attended the National Union Convention in 1866 and was elected to the 39th Congress as an Unconditional Unionist the same year following the readmission of Tennessee into the Union. He then served in the 40th, 41st, 42nd and 43rd Congresses as a Republican. During the 43rd Congress he acted as chairman of the U.S. House Committee on Banking and Currency. In 1868, while serving in Congress, Maynard was appointed to the Tennessee Supreme Court by Governor Brownlow to fill the vacancy created by the resignation of Samuel Milligan. Maynard held both offices simultaneously, but his right to do so was challenged in "Calloway v. Strum", 48 Tennessee (1 Heiskell), 764. The court in that case held that Maynard's judicial acts were valid based on the presumption that his acceptance of the Supreme Court appointment constituted a resignation of his congressional office, and that it was up to Congress to address his continued occupation of that office. Maynard's name was on the ballot to be nominated for Vice President in the 1872 National Union Republican Convention held in Philadelphia on June 5 and 6. Maynard lost and only received 26 total votes. All 24 from Tennessee, and 1 each from Alabama and Mississippi.