Mullen attended Trinity High School of Manchester, New Hampshire, where he helped lead the Pioneers to the state championship in 1988.

[7] After serving as a graduate assistant at Syracuse and Notre Dame, Mullen was hired by Urban Meyer to coach QBs at Bowling Green from 2001 to 2002.

[8] Following an undefeated 2004 season which led to Utes QB Alex Smith being the first overall pick in the draft Meyer was hired by the University of Florida.

Mullen served as interim offensive coordinator[9] for the 2005 Fiesta Bowl and led the team to a 35–7 win over the Pittsburgh Panthers.

Mississippi State capped off the 2010 season by defeating traditional power Michigan in the 2011 Gator Bowl, 52–14, and achieved a number-15 ranking in the final AP poll.

Bo Wallace fumbled in the end zone, giving MSU the 17–10 victory, and for the first time in school history, a fourth straight bowl berth.

He claimed his first Top 10 win at Mississippi State, as the Bulldogs knocked off #8 LSU, 34–29, at Death Valley, allowing them to enter the polls at #12.

[21] Primarily on the strength of his sterling 2014 season, Mullen was already the third-winningest coach in Mississippi State history, behind only Jackie Sherrill and Allyn McKeen.

On November 26, 2017, Mullen signed a six-year, $36.6 million contract[22] to become the new head coach of the Florida Gators football team.

However, they lost their last three games while giving up 144 points, including a 55-20 loss to Oklahoma in Florida's first ever trip to the Cotton Bowl Classic.

After blowout losses to arch-rival Georgia and 18.5-point underdog South Carolina in October, Mullen fired defensive coordinator Todd Grantham.

[28] On May 14, 2022, Mullen began volunteering as a "contributing resource" for the Lake Oconee Academy Titans by AD Chris Ingle.

[29] Rumors flared days before this was released that he would be the offensive coordinator;[30] however, GHSA By-Law #2.51 states "An athletic coach must be a professional teacher meeting the teaching requirements and be employed by only one board of education or similar governing authority.

He worked as a studio analyst and color commentator, covering college football for ABC and ESPN until 2024 when he reentered coaching.