Bell first attracted notice during his sophomore season at USC. in 1974 as a great blocker and between-the-tackles runner, sharing the position of fullback with David Farmer for the 10–1–1 National Championship Trojans team that defeated the Ohio State Buckeyes in the Rose Bowl by the score of 18–17 on January 1, 1975.
In 1975, Bell led the Trojans to a 7–0 start to their season. Then, the lack of their passing game to balance the offense, led to the team's stumbling to an 8–4 overall record, but capped with a victory over Texas A&M in the Liberty Bowl. During this season, Bell led the nation in rushing, gaining 1,875 yards, as he finished third in the voting for the Heisman Trophy.
Bell was the first overall draft choice in the 1977 NFL Draft, selected by the Tampa Bay Buccaneers. Bell signed a five-year contract for a reported $1.2 million. It was by far the richest contract ever signed by an NFL rookie. This draft choice was somewhat controversial because Tony Dorsett was being projected as an arguably better back than Bell. Bell's selection was not a surprise, however, because Tampa Bay was coached by John McKay, Bell's former head coach at U.S.C. After a couple mediocre seasons, in 1979, Bell enjoyed his finest season, rushing for 1,263 yards and leading the Buccaneers to the championship of the NFC Central Division. He led the Buccaneers to their first playoff win in franchise history that season by rushing for 142 yards on 38 carries scoring two touchdowns against the Philadelphia Eagles. The team fell one game short of a trip to Super Bowl XIV, ending their season by losing to the Los Angeles Rams for the NFC championship.
In March 1982 McKay sent him to the San Diego Chargers. But, suffering from weight loss, aching muscles and severe skin problems, he retired before the 1983 season.