
Bennett was born on August 25, 1965, in Birmingham, Jefferson County, to Lino, a retired steelworker, and Lillie Bennett; he had one sibling, a brother. Growing up in the Ensley section of Birmingham, Bennett was a multi-sport athlete in baseball, basketball, and football at Ensley High School. By his senior year in 1982, Bennett was the state's top football prospect, having run for 1,099 yards as a fullback and catching 12 touchdown passes as a tight end that same year. During his senior year, Cornelius was recruited by Coach Paul "Bear" Bryant from the University of Alabama. Bennett signed with the university just a month after Ray Perkins was hired as the new head coach to succeed the retiring Bryant.

In 1987, Bennett was selected by the Indianapolis Colts as the second pick in the NFL Draft, making him the UA's highest draft pick since quarterback Joe Namath in 1965. Soon after graduation, Bennett married fellow Alabamian Tracey; the couple had no children. Bennett refused to report to the Colts training camp over contract disputes and in October was traded to the Buffalo Bills in a complicated series of transactions colloquially known as the "Trade of the Decade" by the New York Times. In it, the Colts traded Bennett to the Bills in exchange for the Bills' first-round draft pick in the 1988 draft, second round selections in the 1989 draft, and a three-way trade in which Bennett went to the Bills in exchange for Bills' running back Greg Bell. He was then traded with three draft choices and running back Owen Gill to the Los Angeles Rams for running back Eric Dickerson, who went to the Colts.
With the Buffalo Bills, Bennett was named to five Pro-Bowl selections in 1988, 1990, 1991, 1992, and 1993 and played in four consecutive Super Bowls from 1990-1993, all of which the Bills lost. He was a starter for the final seven games of the 1987 season and was named to several "All-Rookie" honorific teams and the Sports Illustrated Rookie of the Year. In 1988, he was named by United Press International as the American Football Conference (AFC) Co-Defensive Player of the Year and AFC linebacker of the year. Following the 1989 season, he was awarded the Sington Trophy (named after Fred Sington, who played tackle for the University of Alabama) as the Male Athlete of the Year in Alabama. In 1991, he was an All Pro First Team Selection who played five different positions and led the Buffalo Bills in sacks, quarterback pressures, and forced fumbles and was again named the AFC Defensive Player of the Year. In 1992, he and his wife Tracey divorced.
After eight years with the Buffalo Bills, in 1996 Bennett became a free agent and signed with the Atlanta Falcons, with whom he played until 1999, and made one visit to the Super Bowl, which the Falcons lost. In 1997, he married his wife Kimberly, with whom he would have two daughters and a son. In February 1998, he was sentenced to two months in jail and three years' probation for sexual assault against a female acquaintance. From 1999-2000, Bennett played for the Indianapolis Colts before retiring in 2000 and moving to Florida.
