Patricio Garino scored a season-high 19 points to lead George Washington (16-5, 5-3 Atlantic 10 Conference), which has won five straight against the Patriots. Larsen finished with 10 points and 11 rebounds.
Otis Livingston II scored 18 points for George Mason (7-14, 1-7).
The Patriots used a 7-0 surge to take a 61-59 lead, then Joe McDonald answered with a pair of free throws to tie it. After a Patriots' miss, Larsen made a pair of free throws, grabbed a defensive board, and then tipped in Yuta Watanabe's layup attempt and the Colonials led 65-61 with two minutes left.
Garino forced a turnover on the ensuing possession and scored a layup, and the Colonials then made 9 of 10 free throws, including two from Larsen to seal it.