Maguire was a star player for Delbarton School, helping his team win four consecutive Mennen Cups and win a share of the state championship as a junior. After graduating in 1990, he was selected by the Montreal Canadiens in the NHL Draft and then began attending Harvard University in the fall. Maguire was a more defensive-oriented player in college, playing against tougher opponents, but he was still able to help the Crimson finish atop the ECAC Hockey standings for three consecutive years. In his senior season, Maguire's offensive game came alive and he more than doubled his previous career best. He was named an All-American and pushed Harvard to an ECAC championship and an appearance in the Frozen Four.