He flew on the Apollo 11 mission, the first lunar landing. On this mission Collins was the Command Module pilot who orbited the Moon while his colleagues Neil Armstrong and Buzz Aldrin landed and walked on the lunar surface.
Prior to this, he flew on Gemini 10. On this flight he and his shipmate John Young set a new record for the highest flight, 475 miles above the Earth. Collins walked in space on this mission.
He wrote a book, Carrying the Fire [ISBN 1885283148], about his experiences as an astronaut.