Valenti had 12 points in the first half as Canisius held a 32-30 lead. McMillan scored all but two of his points in the second half, including 8-of-8 shooting from the stripe in the final 1:30.
Matt Scott hit two free throws with 2:40 left to pull Niagara to 57-53. But Kevin Bleeker answered with a 3-point play and McMillan hit two free throws for a nine-point lead.
Kassius Robertson added 15 points for Canisius (10-10, 5-4 Metro Atlantic Athletic Conference), which set a program record with its fourth straight victory in the Gallagher Center.
Scott scored 20 points for the Purple Eagles (5-15, 3-6), who faced Canisius, located just 24 miles away, for the 177th time since 1904.