Saint Peter's (12-16, 8-11 Metro Atlantic Athletic Conference) trailed 60-51 with 3:32 left, before closing on a 12-2 run to beat the Stags on a game-winning 3 from Washington for the second time this season.
Fairfield (6-24, 3-16) turned the ball over four times during the late run, and Saint Peter's closed to within 62-60 on Quadir Welton's layup with 29 seconds remaining. Fairfield's K.J. Rose missed two free throws with 20 seconds left, and Washington's deep 3-pointer capped the comeback.
Marvin Dominique had 12 points and 10 boards for the Peacocks. Jamel Fields added 12 points.
Amadou Sidibe led Fairfield with a career-high 18 points and added 14 rebounds. Maurice Barrow had 15 points.
Washington hit a 3-pointer with a second left on Jan. 8 to down Fairfield 56-55.