Hardin served as a colonel in the 12th Pennsylvania Reserve Regiment. He lost his left arm in the Mine Run Campaign, but continued serving in the army. He was mustered out of the volunteer service in June 1864 but returned to active duty on July 2 with an appointment to brigadier general. General Hardin commanded a division in the XXII Corps during the battle of Fort Stevens.

Martin Davis Mardin House in Lake Forest, Illinois
After retiring in December 1870, he became a lawyer in Chicago, and had a winter home in St. Augustine, Florida. His second wife, Amelia McLaughlin (1863–1939) was the sister-in-law of Irene Castle, the well-known dancer. Hardin spent his last years in the famous "Union Generals' House" at 20 Valencia Street in St. Augustine, Florida (saved from a proposed demolition by Flagler College in the 1980s through the concerted action of local history lovers). Hardin was one of the last surviving Civil War generals of either side at the time of his death in 1923. He is buried, with his wife, at the U.S. National Cemetery in St. Augustine.[2] His widow honored him by building a chapel (complete with a statue of St. Martin) on the grounds of the Mission of Nombre de Dios in St. Augustine.