Reggie Hearn added 20 points on 7 of 8 shooting for the Wildcats (14-8, 4-6), who shot 60 percent from the floor to win back-to-back Big Ten games for the first time this season.
Brandon Paul scored 22 points and Meyers Leonard had 21 for Illinois (16-7, 5-5). Leonard was 10 of 12 from the field, making his first nine shots.
Northwestern opened the game with four quick 3-pointers to take an 18-9 lead, but a driving layup from Tracy Abrams topped a 20-11 run for Illinois to tie the game at 29.
Illinois didn’t hold a lead until 16 minutes left in the second half, and an Abrams jumper with 5:17 left tied the score at 56-56, but Northwestern went on a 16-8 run from there to take control.