Joyner showed great enthusiasm for athletics early on, and as a teenager she won the first of four consecutive National Junior Pentathlon championships. In high school she was a determined student and athlete, graduating near the top of her class and competing on the school's volleyball, basketball, and track teams. As a junior, she set the Illinois high-school girls' long jump record at 6.68 metres (20 feet 7.5 inches). Her high-school success led to a scholarship to the University of California, Los Angeles, where she initially focused on basketball and the long jump.

Joyner began training for the heptathlon in 1981 under the tutelage of assistant track coach Bob Kersee, whom she married in 1986. At the 1984 Summer Olympic Games in Los Angeles, she overcame a pulled hamstring to win the silver medal in the heptathlon, narrowly missing the gold by five points. She graduated from UCLA in 1985, and on July 7, 1986, Joyner-Kersee finally emerged as the dominant heptathlete, setting a world record (7,148 points) at the Goodwill Games in Moscow. Her score bettered the old record by 202 points, making her the first heptathlete to top 7,000 points. Joyner-Kersee set the heptathlon world record (7,291) for the fourth time while winning the gold medal at the 1988 Olympics in Seoul, South Korea. In 1992 in Barcelona, Spain, she became the first athlete to win the heptathlon in consecutive Olympic Games. In her final Olympic appearance at the 1996 Games in Atlanta, Georgia, she earned a bronze medal in the long jump; a hamstring injury forced her to withdraw from the heptathlon.