Born in Cambridge, Ohio, Glenn became a US Marine Corps pilot, flying combat missions in World War II and the Korean War (1950–3). He joined NASA in 1959 and became an early recruit into NASA's Mercury space program. On 20 February 1962, he became the first US astronaut to orbit the Earth, doing so three times in the spacecraft Friendship 7. The flight lasted 4 hours 55 minutes. On 29 October 1998, Glenn became the oldest person in space when, at the age of 77, he embarked on a nine-day mission aboard the shuttle Discovery.

Glenn retired from NASA in 1964 and rejoined the military, but he resigned from the Corps as a colonel in 1965 after an injury. He was elected to the US Senate as a Democrat from Ohio in 1974, and was re-elected in 1980 and 1986.

Glenn unsuccessfully sought the Democratic presidential nomination in 1984. He left the Senate in 1999, the year NASA renamed its Lewis Research Center in Cleveland, Ohio, as the John H. Glenn Research Center. The last remaining Mercury astronaut, Glenn died in Ohio on 8 December 2016 at the age of 95.