Benjamin served during the American Civil War as a colonel in the 43rd Regiment of the New York State Volunteers. Later, he commanded the Light Division of Sixth Corps (part of the Army of the Potomac) as a Brevet Brigadier General. He was noted for his bravery during the charge at Marye's Heights during the Battle of Fredericksburg. After the war, he joined the Loyal Legion, then served as a member of the New York State Assembly from 1880 to 1882.