McDonald, one of four freshmen starting for the Colonials (8-9, 2-2 Atlantic 10 Conference) made two free throws with 13 seconds remaining after the Minutemen (12-5, 2-2) had cut their deficit to 75-74.
Massachusetts’ Freddie Riley sank two free throws with 3 seconds left, but Isaiah Armwood answered with two for the Colonials.
Chaz Williams and Raphiael Putney rallied the Minutemen from a 52-36 deficit with 14:59 left, combining for 26 of Massachusetts’ final 40 points. Williams finished with 18 points, Cady Lalanne 15 and Putney 14.
Dwayne Smith scored 15 points, Lasan Kromah 14 and Patricio Garino 12 for George Washington, which led 40-33 at halftime.
Massachusetts retired the jersey of 1996 Naismith Player of the Year Marcus Camby at halftime.