The senior guard made 12 of 21 shots, including 5 of 7 from 3-point range, coming up one point shy of the 36 he scored in a season-opening loss to Baylor for the Mountain Hawks (2-2).
McCollum scored 21 of his points in the first half, including three consecutive 3-pointers that enabled Lehigh to turn an early 10-7 lead by the Stags (2-2) into a 16-10 Mountain Hawks advantage.
Lehigh led 40-30 at the half and never by fewer than nine after that.
Gabe Knutson added 17 points and Holden Greiner 11 rebounds for Lehigh, which is hosting a consolation bracket of the NIT Season Tip-Off. Mackey McKnight added seven assists.
Keith Matthews scored 17 points and grabbed 10 rebounds to pace Fairfield.