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.
We have changes at the top of the draft and a shakeup in the lottery, with teams starting to make their final decisions. Get the latest in Chad Ford's mock draft.
Duke freshman point guard Derryck Thornton Jr. is transferring to USC, his father announced on Friday to Scout.com.
NBA draft prospect Kris Dunn is excited to see Game 7 between the Cavaliers and the Warriors, he breaks down the constant comparison between LeBron James and Steph Curry and looks forward to playing with or against some of these guys next season.