Surrounded by older students, encouraged by a father who liked to travel, and tutored by a devoted mother, John developed an early interest in science, a fascination with flying, and a sense of patriotism that would define his adult life. He graduated from New Concord High School and attended Muskingum College.
John met Annie Margaret Castor when they were just toddlers. Their parents came to New Concord at about the same time and soon became friends. When they got together for dinner, John and Annie would share a playpen. John and Annie started dating in high school. They were married on April 6, 1943 and still are together to this day.
In March 1942 he enlisted as a United States Navy aviation cadet. He trained at Naval Air Station Olathe, Kansas, where he made his first solo flight in a military aircraft. After completing his training, Glenn was assigned to Marine squadron VMJ-353. He flew 59 combat missions in the South Pacific. He was promoted to Captain shortly before the war ended. In 1948 he became a flight instructor.
Glenn flew to Korea in February 1953, assigned to First Marine Air Wing, VMF-311. He flew 63 ground support missions, and earned 2 Distinguished Flying Crosses and an additional 8 Air Medals.
In April 1959, Glenn was selected as a Project Mercury astronaut. He became part of the Mercury Seven group, the first astronauts selected by NASA. Glenn served as backup astronaut to the first two Americans in space, Alan Shepard and Virgil "Gus" Grissom.
John piloted the Friendship 7 spacecraft, which was launched from Kennedy