Flew 56 combat missions in Europe as a B-25 pilot from October, 1943 to May, 1944. In May 1945, Slayton went to Okinawa with the 319th Bombardment Group and flew seven combat missions over Japan in A-26s.
Attended the USAF Test Pilot School at Edwards Air Force Base, California which he graduated from in December of 1955.
Selected for the Mercury Astronaut mission, but was grounded from flying after being diagnosed with a heart anomaly. He then became the coordinator of astronaut activities.
Appointed Assistant Director of Flight Crew Operations at NASA in 1963 where he was heavily involved in the planning of all the Mercury, Gemini and Apollo missions and personally selected the crews of each mission. He also played a major role in bringing the Apollo 13 crew home safely
Directed the activities of the Astronaut Office, the Aircraft Operations Office, the Flight Crew Integration Division, the Crew Training and Simulation Division, and the Crew Procedures Division.
Served as Docking Module Pilot in the Apollo-Soyuz Test Project (ASTP), a joint American and Soviet space flight, after NASA restored him to full flight status. The event marked the successful testing of a universal docking system and signaled a major advance in efforts to pave the way for joint experiments and the exchange of mutual assistance in future international space explorations.
Was the last Mercury Seven and the oldest astronaut (at the time) to fly into space.
Ran the shuttle Approach and Landing Test (ALT) program for the Space Shuttle program’s orbital test flights in 1975.
Participated in Formula One racing where he raced 43 times in 18 different meets.