Brad Waldow hit 1 of 2 free throws with 18.8 seconds left to give Saint Mary's a 71-69 lead. NAU's Kris Yanku scored to tie the game and after Upshur's heroics, Saint Mary's long heave was off the mark.
Upshur finished with 26 points for Northern Arizona (4-6) and Yanku had 18.
Waldow led Saint Mary's (6-2) with 25 points for his fourth straight double-double and his sixth in eight games.
NAU used a 9-0 run to come back to take a 66-64 with 1:55 left. Emmett Naar ended the Gaels' 6-plus minute field goal drought with a 3-pointer and Aaron Bright hit another from distance on their next possession for a 70-66 lead. NAU's Upshur answered with a 3-pointer to pull within one with 51 second left.