Thomas Lyon Hamer (July 1800-2 December 1846) was a member of the US House of Representatives (D-OH 5) from 4 March 1833 to 3 March 1839, succeeding William Russell and preceding William Doan. Born in Northumberland County, Pennsylvania, he worked as a teacher before becoming a lawyer in Georgetown, Ohio and serving in the state house from 1825 to 1826 and from 1828 to 1830 and in Congress from 1833 to 1839. During the Mexican-American War, he became a brigadier-general of volunteers and fought at the Battle of Monterrey. He fell ill and died at Monterrey during the war, and his constituent Ulysses S. Grant lamented the loss of a potential future president.