Shortly before her fifteenth birthday, Carpenter finished seventh in the 1,500-meter speed skating event at the 1972 Olympics. She won the U.S. outdoor over-all championship in 1976, but injured an ankle and missed the Olympics.
Like most speed skaters, Carpenter trained on a bicycle during the off-season. After her ankle injury, she began cycling competitively and won the national road and pursuit championships in 1976, 1977, and 1979.
Carpenter suffered a concussion in a fall and temporarily quit cycling to return to the University of California-Berkeley, where she took up rowing. She was a member of the crew that won the 1980 national collegiate championship in the four-oared shell with coxswain.
Carpenter returned to cycling in 1981 and won the national road and two-points championships that year. She was also the national two-points champion in 1982 and criterium champion in 1982 and 1983. Carpenter set a world record of 3:49.53 in winning the 1983 world pursuit championship. The following year, she became the first U. S. cyclist since 1912 to win an Olympic medal.
Rebecca Twigg of the U. S. took the lead in the Olympic road race with 50 meters to go, but Carpenter pulled even just 3 meters from the tape, then eased out of the saddle and threw her arms forward to propel her bike across the finish line less than half a wheel length ahead of Twigg. It was a trick she'd learned from her husband, Davis Phinney, who won a bronze in the team time trial.
Afterward, Carpenter said that the crowd of 200,000 people that lined the course in suburban Los Angeles definitely helped U. S. cyclists. "What made the Olympics special," she said, "was that a number of us had raced in the world championships several times where the support wasn't there, so we appreciated the support in the Olympics."