As a football player, he played linebacker for the Miami Hurricanes from 1985 to 1988 followed by two seasons in the NFL with the Dallas Cowboys, all under head coach Jimmy Johnson.
[3] Shannon attended Miami Norland High School, where he earned All-state honors playing football as a defensive back in his senior year.
The next year as a backup, he tallied 82 tackles (fifth on the team), including 13 against the University of Pittsburgh that was led by running back Craig Heyward.
He was named a starter in the third game of the season against the Washington Redskins, becoming the first Cowboys rookie to start at outside linebacker since Dave Edwards did it in 1963.
[8] Miami Hurricanes head coach Dennis Erickson hired Shannon in 1991 to be a graduate assistant at his alma mater.
That year Miami won the BCS National Championship, and Shannon received the Broyles Award recognizing him as the best assistant coach in college football.
[9] Coker stayed on to coach the team to a 21–20 MPC Computers Bowl victory over the University of Nevada; Shannon assumed all other functions, including recruiting, immediately upon his hiring.
[10] Shannon's first decision as head coach was to remove the players' surnames from their jerseys, in order to emphasize the team over individual members.
[11] He also instituted a strict code of conduct for the program, a response to the frequent on-field and off-field misconduct that characterized Coker's latter tenure.
Shannon expressed frustration over the media's handling of such incidents, stating that the coverage made Miami look like a haven for crime.
The 'Canes briefly returned to the Top 25 rankings for the first time since early in the 2006 season before surrendering 472 rushing yards to Georgia Tech in a 41–23 late-November loss that eliminated Miami from ACC Championship contention.
In the immediate aftermath of the bowl game, Shannon fired his offensive coordinator, Patrick Nix, over philosophical differences.
Nix wanted to employ more of a spread attack, whereas Shannon remained committed to Miami's traditional pro style offense.
Shannon eventually hired former Pittsburgh Steelers and Philadelphia Eagles assistant coach Mark Whipple for the position.
Nix's departure was followed by news that Robert Marve, a redshirt-freshman quarterback who had been suspended from the bowl game for repeatedly missing class, asked for a release to transfer to another school.
[16] Marve cited a strained relationship with Shannon, who had previously suspended him after his arrest for criminal mischief, as his reason for leaving.
[18] Shannon's staff suffered more upheaval when defensive coordinator Bill Young left to assume the same position at Oklahoma State, his alma mater, in late January.
Though they were ranked in the top-25 until the final two games of the season, the season-ending loss to South Florida left them unranked heading into the Sun Bowl.
Perhaps most significantly, he was apparently untainted by the scandal that engulfed the program in the 2011 season, as he avoided contact with Nevin Shapiro, the rogue booster who admitted to providing massive amounts of improper benefits to Miami players from 2002 to 2010.
Sports Illustrated writer Alexander Wolff, in an August 2011 open letter to university president Donna Shalala, noted that Shannon "seems to have been the only person in Coral Gables who wanted nothing to do with Shapiro, reportedly warning his players to avoid him and threatening to fire assistants caught dealing with him.
[22][23] Three members of his second recruiting class—Marcus Forston, Sean Spence and Marcus Robinson—were recognized by College Football News as freshman All-Americans.
[31] Collins accepted the head coaching position at Temple in December 2016, and Shannon was promoted to be Florida's full-time defensive coordinator for the 2017 season.
[35] In April 2021, Shannon was hired to be the senior defensive analyst for the Florida State Seminoles football team quickly helping as an assistant linebackers coach as well.