Cernan was commissioned in the U.S. Navy in 1956, became a test pilot, and earned his master's degree in aeronautical engineering at the U.S. Naval Postgraduate School, Monterey, Calif. In 1963 he was named in the second group of astronauts.

Cernan and Thomas P. Stafford (Stafford, Thomas P.) were launched into space on June 3, 1966, in Gemini 9. In addition to the activity outside the craft, they rendezvoused three times with a target vehicle during the three-day mission.

On May 18, 1969, Cernan, Stafford, and John W. Young (Young, John W.) began the eight-day mission of Apollo (Apollo program) 10. As Lunar Module pilot, Cernan brought the landing craft into a close lunar orbit, approaching the surface to within 16 km (10 miles). Stafford and Cernan completed a complex series of orbital maneuvers before rejoining the Command Module. The mission performed every function necessary for a lunar landing but the landing itself and was the final test of Apollo (Apollo program) systems.

Cernan commanded the Apollo 17 Moon flight (with Ronald Evans and Harrison Schmitt, Dec. 7–19, 1972). He and Schmitt, a geologist, explored the Taurus-Littrow region of the Moon's surface (Dec. 11–14) and concluded the Apollo Moon program. After serving as deputy director of the Apollo-Soyuz Test Project (completed in July 1975), Cernan resigned from the navy and the space program in 1976 to enter private business.