Marc Thompson attended Kansas State University, where he ran track and cross-country, graduating in 1975. He had a brief cycling career that started in 1973 and ended in 1978. Thompson placed third in the road race at the 1975 US Championships, which put him on the US team at the 1975 Pan American Games, where he finished fourth with the US team in the road time trial. He also competed at the 1974 World Championships in the road race. In 1978 Thompson left cycling to return to running, and compete in what was then the new sport of triathlon. He ran a 2-25 marathon in 1981 at the Houston Marathon, and finished 72nd at the 1982 Hawai’i Ironman Triathlon. Thompson improved greatly in 1983, placing fourth at the Hawai’i Ironman. He later became a triathlete coach.