At the start of the American Civil War, he assisted in raising and equipping a unit which was mustered into service as Company H, 34th New York Volunteer Infantry. He later helped raise a cavalry company, which he joined as a private; this unit became Company H, 5th New York Volunteer Cavalry Regiment, and the members elected Hammond to be their commander with the rank of captain. During the war he took part in several battles, including the Second Battle of Bull Run and the Battle of Cedar Mountain. He was wounded twice, and advanced to become commander of the 5th New York Cavalry with the rank of colonel. After the war he received the brevet rank of brigadier general in recognition of his superior performance of duty. Hammond became active in the Grand Army of the Republic, and the Military Order of the Loyal Legion of the United States, and the GAR post in Crown Point was named for him.