Following the death in action of his brother Jack, a member of the United States Army's Third Division, in 1944, Sinnott acceded to his mother's wishes not to be drafted into the Army himself, and he enlisted in the U.S. Navy in the autumn of that year. After serving with the Seabees in Okinawa during World War II, driving a munitions truck, he was discharged in May 1946 and awarded the Asiatic–Pacific Campaign Medal, the World War II Victory Medal, and the Navy Occupation Service Medal. After working three years in his father's cement-manufacturing plant, he was accepted into the Cartoonists and Illustrators School (later the School of Visual Arts) in New York City in March 1949, attending on the GI Bill.