Kelly reportedly interviewed for the Florida job a week ago but it's unclear if he ever had a formal offer from the Gators.
Mullen spent nine seasons in Starkville, going 69-46 with seven winning seasons and a pair of Top-25 finishes.
Spurned by Chip Kelly and seemingly unwilling to risk waiting for UCF's Scott Frost to possibly do the same, Florida athletic director Scott Stricklin turned to Mullen.
Mullen was the Gators' offensive coordinator from 2005-08, including the 2006 and 2008 National Championship teams, coaching Heisman Trophy victor Tim Tebow. But he did a commendable job in Starkville, especially when you see the progress quarterbacks Dak Prescott and Nick Fitzgerald made on his watch.
The Gators went 4-7 this season for just their second losing season since 1979.
At Mississippi State, Mullen worked under Stricklin for five years before the athletics director left to take over at UF last fall.
The Bulldogs are 8-4 overall and 4-4 in the SEC this season.
Mullen replaces Jim McElwain, who was sacked in midseason.
One way or another Florida will either be set to hire Mullen, or be set to move on to another candidate (Scott Frost?), before the end of the day.