Larsen and McDonald each made five shots from the field while Lasan Kromah, who had led George Washington (3-3) in scoring the past two games, finished with 10 points on 4-of-6 shooting.
The game went back-and-forth, as neither team led by more than four. Charles Cooke broke the games' seventh tie with a free throw to put James Madison (1-5) up 48-47 with 3:12 left.
But the Dukes would not see the lead again, as Larsen hit a layup to help George Washington retake the lead, 52-48, and Patricio Garino hit a 3-pointer to put the Colonials ahead for good.
James Madison scored just one basket over the final two minutes before Andre Nation sank a desperation heave at the buzzer.
The dust has settled after players made their NBA draft decisions. Which college teams won? Which lost? Well, Villanova is pretty happy. USC, meanwhile, might not be as thrilled.
The 2017 recruiting class is short on point guards, with Trevon Duval leading only eight prospects from the ESPN 60 who are still available.
ESPN college basketball reporter Andy Katz looks into the two decommitments from the NBA draft and what they're looking forward to.