Matt Howard scored 14 and Khyle Marshall added 10 for the Bulldogs (27-9), who showed again they simply won't give in, erasing an 11-point hole in the second half.
Mack's 3-pointer with 1:21 left in overtime gave Butler the lead for good at 72-70.
Kenny Boynton missed a long 3 that could have given Florida (29-8) the lead with a little under 20 seconds left. Alex Tyus appeared to have the offensive rebound, but Howard tied him up and the possession arrow favored Butler.
Florida had to foul Mack with 10.6 seconds to go, and he hit both shots for the final margin.
Vernon Macklin had a career-high 25 points for fourth-seeded Florida, the third-straight higher seed from a major conference to be vanquished by the Bulldogs.
Boynton finished with 17 points and Tyus had his second-straight double-double with 14 points and 10 rebounds.
Butler had to overcome Florida's size advantage and a number of its own mistakes.
In regulation, the Bulldogs made 10 of 20 free throws and shot 39.6 percent (21 of 53), including 8 of 30 from 3-point range. Yet they somehow found a way to survive to overtime, when they hit all seven foul shots and three of their eight field goals.
Remarkably, Butler also outrebounded the Gators, 41-34.
Now the Bulldogs are one win away from a second straight appearance in the national championship game.
Gators Fall to Butler in OT
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