Thomas first competed as a member of the U.S. Olympic team at the 1976 Summer Olympics. Two years later, he became the first American male gymnast to win a gold medal in floor exercise, accomplishing the feat in the 1978 World Championships. He subsequently became the first gymnast to receive the James E. Sullivan Award for the best amateur athlete in the United States. Thomas earned six medals at the 1979 World Championships, including gold on the horizontal bar and floor exercise, and silver in the all-around, parallel bars, and pommel horse, establishing a new American record for most medals won at a single worlds. The feat would later be tied by Simone Biles in 2018. He was seen as a favorite to win a gold medal at the 1980 Summer Olympics in Moscow; however, the games were boycotted by the United States government in protest of the Soviet invasion of Afghanistan.