As an officer in the reserves, McMahon was recalled to active duty during the Korean War. He flew an OE-1 (the original Marine designation for the unarmed single-engine Cessna O-1 Bird Dog) spotter plane, serving as an artillery spotter for Marine batteries and a forward controller for Navy and Marine fighter bombers. He flew a total of 85 combat missions, earning six Air Medals. After the war, he stayed with the Marines as a reserve officer, retiring in 1966 as a colonel. In 1982, McMahon received a state commission as a brigadier general in the California Air National Guard, an honorary award to recognize his support for the National Guard and Reserves.