In the era before freshman eligibility, Cappelletti was a running back on the freshman team at Penn State in 1970. During his sophomore season in 1971, he played as a defensive back, as the Nittany Lions had two senior running backs who were taken early in the 1972 NFL Draft: Franco Harris and Lydell Mitchell. As a senior tailback at Penn State in 1973, Cappelletti gained 1,522 yards on 286 carries scoring 17 touchdowns as the Nittany Lions rolled to an undefeated 12–0 season. He was awarded the 1973 Heisman Trophy, Maxwell Award, the UPI College Football Player of the Year, the Walter Camp Award, the Chic Harley Award, as well as receiving All-America honors. In his two-year running-back career, Cappelletti gained over 100 yards in thirteen games and had a career total of 2,639 yards and twenty-nine touchdowns for an average of 120 yards per game and 5.1 yards per carry. His Heisman acceptance speech, where he dedicated his award to his dying brother Joey, is one of the most memorable in the history of college sports. The relationship between Cappelletti and his younger brother, who died of childhood leukemia on April 8, 1976, was made into a television movie in 1977 called Something for Joey; Cappelletti was played by Marc Singer. The movie was based on the book of the same name written by Richard E. Peck and chronicled the bond between the two brothers as Cappelletti supported his young brother, ill with cancer. During Cappelletti's senior season, Penn State played West Virginia in late October. The morning of the game, Cappelletti asked Joey what he wanted for his upcoming 11th birthday. Joey replied "I want you to score three touchdowns for me. No, four." In Something for Joey, a shocked Cappelletti is seen confiding to a teammate: "How am I going to score four touchdowns?" At the end of the first half, Cappelletti had scored 3 touchdowns, well on his way to four. But head coach Joe Paterno did not like to run up the score against opponents, so when the game resumed after halftime, Paterno told Cappelletti he would be on the bench. Cappelletti quietly took his seat on the bench, without telling Paterno of Joey's wish. Late in the third quarter, one of Cappelletti's teammates told Paterno of Joey's wish. On Penn State's next possession, Paterno shouted "22" and Cappelletti took the field; he scored his fourth touchdown on the same possession, and pointed to Joey as he ran off the field. The Lions scored three more touchdowns in the fourth quarter and won 62–14.
Cappelletti was the eleventh overall pick of the 1974 NFL Draft, taken by the Los Angeles Rams. He played nine seasons in the league, five with the Rams, and four with the San Diego Chargers He missed the entire 1979 season due to a nagging groin injury.
