During the Civil War, Smith was a member of the 1st Maine Volunteer Cavalry Regiment. He was commissioned a captain in November 1861 and took command of Company D. He served well and was promoted to major on February 16, 1863. Promoted again March 26 to lieutenant colonel, he participated in the Stoneman 1863 raid. He commanded the regiment in Col. Douty's absence at Brandy Station assuming command after Douty's death at Aldie June 17, 1863 when he was promoted colonel. He was breveted to brigadier general on August 20, 1864, and took command of the brigade. He ended the war at Appomattox having commanded several brigades and a division.