In February 1862, during the American Civil War, he volunteered for the Union Army. He was commissioned as a major in the 100th New York Infantry Regiment, and in October 1862 he was promoted to lieutenant colonel. He commanded the unit in the taking of Folly Island and was brevetted brigadier general for his meritorious service. He was discharged in June 1863.[3] Otis died in Cuba, New York, where he was living at the time, on January 22, 1883.[7]