He was offensive coordinator of other NFL teams, and as head coach, general manager, and president of the Las Vegas Locomotives of the United Football League (UFL).

Three weeks after the Giants won Super Bowl XXV, he was hired by Bill Parcells as their quarterback coach.

[10] The departure of Dan Reeves as coach after the 1996 season led to close consideration between bringing back Parcells or promoting Fassel.

[11] Fassel was hired as the head coach of the New York Giants, starting with the 1997 season, and remained in that position for seven years.

[12][13] During Fassel's tenure as head coach of the Giants, his teams were known for numerous strong runs in December and for winning big games, such as handing the Denver Broncos their first loss of the 1998 season after a 13–0 start.

In his fourth year as head coach, the 2000 season, he received acclaim for his "playoff guarantee" during which he led the Giants to an improbable NFC Championship.

Under heavy criticism from the New York media and Giants' upper management, Fassel ad hoc'ed a famous speech that predicted a playoff berth that proved to be the impetus for a run at Super Bowl XXXV: "This is a poker game, and I'm shoving my chips to the middle of the table, I'm raising the ante, and anybody who wants to get in, get in.

With the NFL games that week suspended, Fassel was called by Mayor Rudy Giuliani to help morale at the Trade Center site.

Fassel agreed and insisted that the team use its goodwill to help the recovery effort and provide assistance to the FDNY, NYPD and the City of New York.

[20] Fassel returned to the Locos in 2010 and helped lead the team to repeat as champions, again defeating the Tuskers in the 2010 UFL Championship Game.

The Locos tried to three-peat in 2011, but this time fell to the Tuskers (who had since been relocated and renamed the Virginia Destroyers) in the 2011 UFL Championship Game.

Fassel entered broadcasting following his firing as offensive coordinator for the Ravens, joining Westwood One radio as a color commentator for its Sunday NFL action.

They had a family reunion, which included the Fassels and their other four adult children, together with their now 34-year-old adopted son, John Mathieson, who brought along his own wife and their four young daughters.

[26] Fassel was good friends with fellow coach Mike Holmgren, dating to their days as USC quarterbacks.