Jenner began his athletic career at Newton (Connecticut) High School, where he was a letterman in football, basketball, and track, and on his own he took up waterskiing, becoming East Coast all-over champion in 1966, 1969, and 1971. He entered Graceland College (Lamoni, Iowa) in 1969 on a football scholarship, but an injury to his knee restricted him to basketball and track. He competed in his first decathlon in 1971 and finished 10th in the decathlon at the 1972 Olympic Games in Munich, West Germany. Thereafter he moved to California, where he trained with individual-event stars in the San Jose area rather than with other decathlon athletes.

After his Olympic triumph in 1976, Jenner became active as a network television sports commentator and made television commercials, lectured, and published several books. He won the James E. Sullivan Trophy as the outstanding amateur athlete of 1976.