Nash enlisted in the Union Army on September 10, 1861, mustering into Company D of the 27th Regiment Massachusetts Volunteer Infantry ten days later. Nash stayed in the Army for the remainder of the American Civil War, re-enlisting when his initial term of enlistment expired in 1863. Through the course of the war, he participated in twenty battles, and was eventually promoted to the rank of Sergeant. Nash was wounded in action at Petersburg on July 16, 1864. On March 8, 1865, Nash was captured in North Carolina, and thereafter briefly held as a prisoner of war until he was exchanged on March 27. Following the end of the war, Nash was mustered out of the Army on June 26, 1865.