Afflicted with anemia in boyhood, Mathias developed strength by engaging in sports, winning success as a teenager in football and basketball. At the suggestion of his high school track coach, Mathias entered his first decathlon competitions in 1948. At the Olympic Games in London that year, Mathias, despite leading in the shot put and high jump, was in third place after the first day of the decathlon. The next day a strong discus throw of 44 metres (144 feet 4 inches) put him in first place, and he went on to win the gold. With his victory, Mathias became the youngest athlete to win a gold medal in an Olympic track-and-field event. In 1948 he received the James E. Sullivan Award as the top American amateur athlete.

Mathias won four U.S. decathlon championships (1948–50, 1952). He attended Stanford University (B.A., 1953), where he played fullback on the gridiron football team. (Although drafted by the Washington Redskins, Mathias never played professional football.) At the 1952 Olympics in Helsinki, Fin., he won the decathlon easily, despite an injured thigh muscle, setting a record of 7,887 points and taking first place in 5 of the 10 decathlon contests. Altogether he entered and won 11 decathlons in his career.

Mathias later performed on television and in films, starring in The Bob Mathias Story in 1954. In 1967–75 he represented California in the U.S. House of Representatives. He was director of the U.S. Olympic Training Center in Colorado Springs, Colo., from 1977 until 1983, the year he was inducted into the U.S. Olympic Hall of Fame.