Penn (19-12, 11-3) needed to win the game to tie Harvard (26-4, 12-2) for the regular-season championship and force a one-game playoff for the league's automatic berth in the NCAA tournament.
Princeton (19-11, 10-4) spoiled those hopes, jumping to a 17-point, first-half lead and withstanding a run that got the Quakers within 34-31 with 12:25 to play.
Hummer then hit a 15-footer to ignite a 7-0 run and the Tigers, who beat Harvard in a playoff for the title last season, closed out the game for their 17th straight league win at home.
Douglas Davis added 12 points and Patrick Saunders had 10 for Princeton.
Zack Rosen had 19 points on 8-of-24 shooting from the field for Penn.