He enlisted in the U.S. Army Air Forces during World War II, deputy commanding the 2nd Bombardment Wing and commanding the 703d Bombardment Squadron from 1941 to 1947. He later transferred to the Air Force Reserve, and held various command positions until his retirement in 1968 as a brigadier general. Stewart remained unmarried until his 40s and was dubbed "The Great American Bachelor" by the press. In 1949, he married former model Gloria Hatrick McLean. They had twin daughters, and he adopted her two sons from her previous marriage. The marriage lasted until Gloria's death in 1994, and Stewart died of a pulmonary embolism three years later.