Lance Crawford scored a season-high 18 points, including 3 of 6 3s, and the River Hawks (10-13, 4-6 America East) hit 9 of 22 from distance (40.9 percent).
There were two first-half lead changes and a tie; the Retrievers led 34-33 behind Malcom Brent and Cody Joyce, who scored 11 apiece, but Harris made a layup at the buzzer and the River Hawks led 35-34 at halftime.
The River Hawks opened the second half with a 15-8 run and led 50-42 after a 3-pointer by Harris, then scored nine while the Retrievers went scoreless for 6 1/2 minutes. Crawford hit two free throws and a layup with 1:30 left for a 20-point River Hawks lead.
Brent led UMBC (3-19, 1-8) with 15 points and Joyce had 14.