Gannon is a former NFL quarterback who achieved most of his success late in his career with the Oakland Raiders.

Gannon attended Saint Joseph's Preparatory and won three varsity letters each in football and crew, and twice in basketball. Gannon attended the University of Delaware - when he led the Raiders to  Super Bowl XXXVII in 2003. He was the first player from the University of Delaware to go to the Super Bowl.

He was selected in the 4th round (98th overall) of the 1987 NFL Draft by the New England Patriots, who envisioned converting him to a defensive back. Gannon balked at the idea, and he was quickly traded to the Minnesota Vikings. After two years of playing sparingly, Gannon became the Vikings' starting quarterback in 1990, until he was passed over for Sean Salisbury towards the end of the 1992 season. In 1993 Gannon was released and signed with the Washington Redskins after coming off rotator cuff surgery. Gannon started three games for Washington and was released at season's end.

Following a hiatus from football for the 1994 season, he signed with the Kansas City Chiefs in 1995. For two years he served as a backup to Steve Bono. In 1997 a quarterback controversy was created when the Gannon-led Chiefs excelled in the absence of the injured Elvis Grbac. In the playoffs, coach Marty Schottenheimer elected to play Grbac instead of Gannon and the Chiefs lost 14-10. The two ended up splitting snaps in 1998, after Grbac was injured in Week 1.

In February 1999 he was signed as a free agent by the Oakland Raiders. He excelled in Jon Gruden's west coast offense and was voted to the Pro Bowl in his first year as a Raider, the first of four straight selections. In 2001 and 2002, he won the Pro Bowl MVP award consecutively, a feat achieved by no other NFL player. In fact, in a Week 2 game against Pittsburgh, Gannon was 43 of 64 for 403 yards, a touchdown and 2 interceptions. Gannon won the NFL Most Valuable Player Award after a record-setting 2002 season, throwing for 4,689 yards and 26 touchdowns, which helped the Raiders advance to Super Bowl XXXVII. He led the league in with 418 completions on 618 attempts.

A serious neck injury in 2004 effectively ended his career. On August 6, 2005 Gannon officially retired from football and joined CBS Sports as an NFL analyst. The same year Gannon was inducted into the University of Delaware athletics Hall of Fame, Gannon retired as an Oakland Raider.