A licensed amateur pilot, Stewart enlisted as a private in the Army Air Corps soon after the United States entered the Second World War in 1941. After fighting in the European theater, he attained the rank of colonel and had received several awards for his service. He remained in the U.S. Air Force Reserve and was promoted to brigadier general in 1959. He retired in 1968 and was awarded the United States Air Force Distinguished Service Medal. President Ronald Reagan would later promote Stewart to the rank of major general in the Air Force retired list, in 1985.