Mills was coached by Marta and Béla Károlyi in Houston, Texas. She attended her first U.S. Nationals in 1984, finishing eighth in the junior division. While Mills was sometimes overshadowed in the media by her teammate Kristie Phillips, she developed a reputation as a steady, consistent competitor in national and international meets, placing third at the 1985 City of Popes competition and second at the 1986 U.S. Olympic Festival and U.S. Nationals. In 1987 she attended her first and only World Gymnastics Championships as a member of the sixth-place American team. She was unable to compete in the 1987 Pan American Games because of an injured heel. 1988 was Mills' breakout year. She won every meet she entered, including the U.S. Nationals, the American Cup, the Mardi Gras Invitational and the International Mixed Pairs, was named the United States Olympic Committee's Gymnast of the Year and was nominated for the prestigious James E. Sullivan Award. She also placed first at the U.S. Olympic Trials, easily earning a spot on the American squad for the 1988 Olympics in Seoul. Mills emerged as the U.S. team's most successful gymnast at the Olympics. While she finished a modest fifteenth place in the all-around, she also qualified for three event finals, more than any other member of the team. She won a bronze medal on the balance beam, behind Daniela Silivaş and Elena Shushunova. With her third-place finish, Mills became the first individual American female gymnast to win a medal in a fully attended Olympics, and the only American gymnast, male or female, to medal in Seoul. Mills continued to compete in early 1989, but retired later that year, due in part to the effects of Epstein-Barr syndrome. She has occasionally returned to the sport as a coach. In 2000, Mills was inducted into the USA GymnasticsHall of Fame.
Diving
After retiring from gymnastics, Mills became involved with competitive diving, specializing in the 10-meter platform event. She joined the diving team at the University of Miami in Florida, where she won the Big East Conference for three years. She also competed in national diving events, participating in the 1993 Olympic Festival and the U.S. Diving Championships. Mills retired from diving after graduation.
Snowboarding
Most recently, Mills has been involved with the sport of snowboarding. She has acted as a snowboarding coach and instructor, working with the U.S. junior national team. Mills served as a judge at the 2014 Sochi Olympic Games.
