When the Japanese attack on Pearl Harbor brought the United States into World War II, Glenn quit college to enlist in the U.S. Army Air Corps. However, he was never called to duty, and in March 1942 enlisted as a United States Navy aviation cadet. He went to the University of Iowa for preflight training, then continued on to NAS Olathe, Kansas, for primary training. He made his first solo flight in a military aircraft there. During his advanced training at the NAS Corpus Christi, he was offered the chance to transfer to the U.S. Marine Corps and took it.
Upon completing his training in 1943, Glenn was assigned to Marine Squadron VMJ-353, flying R4D transport planes. He transferred to VMF-155 as an F4U Corsair fighter pilot, and flew 59 combat missions in the South Pacific. He saw combat over the Marshall Islands, where he attacked anti-aircraft batteries on Maloelap Atoll. In 1945, he was assigned to NAS Patuxent River, Maryland, and was promoted to captain shortly before the war's end.
Glenn flew patrol missions in North China with the VMF-218 Marine Fighter Squadron, until it was transferred to Guam. In 1948 he became a flight instructor at NAS Corpus Christi, Texas, followed by attending the Amphibious Warfare School.
Glenn's Sabre jet
During the Korean War, Glenn was assigned to VMF-311, flying the new F9F Panther jet interceptor. He flew his Panther in 63 combat missions, gaining the nickname "magnet ass" from his alleged ability to attract enemy flak. On two occasions, he returned to his base with over 250 holes in his aircraft. For a time, he flew with Marine reservist Ted Williams, a future Hall of Fame baseball player for the Boston Red Sox, as his wingman. He also flew with future Major General Ralph H. Spanjer.
Glenn flew a second Korean combat tour in an interservice exchange program with the United States Air Force, 51st Fighter Wing. He logged 27 missions in the faster F-86F Sabre and shot down three MiG-15snear the Yalu River in the final days before the ceasefire.
For his service in 149 combat missions in two wars, he received numerous honors, including the Distinguished Flying Cross (six occasions) and the Air Medal with eighteen award stars.