Born in Cambridge, Ohio, he obtained a BS in Engineering from Muskingum College. He enrolled in the Naval Aviation Cadet Program in 1942, and was assigned to the Marines VMO-155 group in 1944. Glenn flew a Corsair over the Marshall Islands, specifically Maloelap, where he was tasked with attacking anti-aircraft gunnery and ground bombardment. By 1945, he was transferred to the Patuxent River Test Pilot School, where he became a test pilot and was promoted to Captain by the war's end.
After the war, Glenn flew patrol missions in North China, based in Guam, but in 1948 he became an flight instructor at Corpus Christi, Texas, then undertook an amphibious warfare course and was assigned a staff assignment, all the while angling to get transferred to combat in Korea. Once there, after flying combat missions for the marines, Glenn got the chance to fly with the Air Force on an inter-service exchange. Flying an Air Force F-86 Sabre, he shot down three MiGs. He received several medals for his service.
He returned to Patuxent River after the Korean war, where he completed the first supersonic transcontinental flight on July 16, 1957 in a Vought F8U "Crusader". The flight was from California to New York and lasted 3 hours, 23 minutes and 8 seconds.
In 1958 Glenn joined NASA as one of the original group of astronauts for the Mercury program, and flew the first American manned orbital mission termed "Friendship 7" on February 20, 1962. He completed three orbits, the mission lasting approximately four hours.
Glenn worked for NASA until 1964, before entering the business world.
In 1974, he entered politics and represented Ohio for the Democratic Party in the Senate between 1974 until finally retiring in 1999. He mounted a bid to be the 1984 Democratic presidential candidate, withdrew from the race.
Glenn lifted off for a second space flight on October 29, 1998 on Space Shuttle Discovery's STS-95. His age of 77 made him the oldest person ever to go into space. Glenn's presence on the nine-day mission was widely criticised by many in the space community as an expensive junket for one of NASA's Congressional supporters.
Glen married his childhood sweetheart, Anna Margaret Castor, and had two children.