McDivitt joined the U.S. Air Force in 1951 and flew 145 combat missions in Korea. In 1959 he graduated first in his engineering class at the University of Michigan, Ann Arbor. He was an experimental test pilot at Edwards Air Force Base, California, when he was chosen as an astronaut in 1962.

McDivitt was the command pilot of Gemini 4 (launched June 3, 1965); a flight that included the first space walk by a U.S. astronaut, and was commander of Apollo (Apollo program) 9 (launched March 3, 1969). He was then manager of the Apollo spacecraft program until he retired from the Air Force in 1972 with the rank of brigadier general and entered private business.