Whack missed the front end of a 1-and-1 with 28 seconds left but made 4 of 4 free throws from there to seal the win for Mount St. Mary's (15-16). The Mountaineers advance to face top-seeded Robert Morris (21-12) in the Northeast Conference championship on Tuesday.
The Mount took the lead for good during an 8-0 run to finish the first half ahead 35-31. Wagner (19-12) closed to within one point at 49-48 midway through the second half, and again at 71-70 with 1:04 left in the game.
Sam Prescott, who finished with 16 points, made two free throws and Whack hit two of his final four free throws to extend the Mount's lead to 75-70 before Kenneth Ortiz's layup cut it to 75-72 with 6 seconds left.
Ortiz had 26 points for Wagner.