Jenner placed third in the decathlon at the 1972 U.S. Olympic trials and finished in tenth place at the 1972 Munich games. His success prompted him to devote himself to an intense training regimen, while also selling insurance outside training hours. In the era before professionalism was allowed in athletics this kind of training was unheard of. During that period he spent eight hours a day at the San Jose City College track. Centered around Bert Bonanno, the coach at SJCC, San Jose at the time was a hotbed for training aspiring Olympic athletes, including Jenner, along with Millard Hampton, Andre Phillips, John Powell, Mac Wilkins, Al Feuerbach and others. In 1974 and 1976, Jenner was the American champion in the event.
At the 1976 Summer Olympics in Montreal, he won the gold medal in the Decathlon, setting the world record of 8,616 points. The world record was broken by just 4 points by Daley Thompson in 1980. In 1985, the IAAF Decathlon scoring table was changed, so Jenner's winning score has been reevaluated against that table and reported as 8634 for comparative purposes. As of 2011, Jenner is #25 on the world all-time list and the #9 American.
As a result of winning the Olympic decathlon, Jenner was a national hero. He was the 1976 recipient of the James E. Sullivan Award as the top amateur athlete in the United States. Jenner was also the Associated Press Male Athlete of the Year in 1976. He was inducted into the Olympic Hall of Fame in 1986 and the Bay Area Sports Hall of Fame, the Connecticut Sports Hall of Fame in 1994 and the United States National Track and Field Hall of Fame in 1980. He was inducted into the San Jose Sports Hall of Fame in 2010.