The teams will meet again Friday on George Mason's home floor to determine to title in the College Basketball Invitation. Santa Clara won the opening game in the best-of-three series 81-63.
George Mason scored eight of the game's first nine points, opened a 39-26 halftime lead and held off a charge in the second half while improving its record to 22-15.
Allen finished with 15 points. Sherrod Wright added 20 points and 10 rebounds for the Patriots.
Santa Clara (25-12) was led by Kevin Foster with 16 points.
Both teams finished fourth during the regular season in their league races -- George Mason in the Colonial Athletic Association and Santa Clara in the West Coast Conference.