Johnny Williams added 14 points for the Patriots (6-6), who trailed the entire game until Allen's jumper with 2:42 remaining.
Saint Mary's cut the deficit to one on two occasions in the final 14 seconds, the last on Walker's free throw with 7.7 seconds that made it 63-62.
Allen then made two free throws to give George Mason a 65-62 lead with 6.2 seconds remaining. Holt was then fouled at midcourt with 3.3 seconds, but only made of his two free throws. Okoloji was fouled, but missed both of his attempts.
Waldow's full-court heave with 1 second remaining went off the backboard.