Mason hit two 3-pointers early in the second half as Yale opened with a 15-5 spurt for a 53-37 lead.
Henry Caruso's driving layup pulled Princeton to 75-72 with 1:30 left, but Mason dribbled down the clock and sank a 3-pointer from straight away for a six-point lead.
Devin Cannady banked in a 3-pointer to get Princeton within 78-75 and the Tigers elected not to foul which led to a Yale miss. Princeton came up empty on its next possession and Yale's Anthony Dallier hit 1 of 2 free throws with 15.6 seconds left for a two-possession game.
Brandon Sherrod scored 19 points on 8-of-8 shooting for Yale (13-5, 4-0 Ivy).
Caruso led Princeton (12-5, 2-1) with 26 points. Cannady added 15 points.