The Black Bears led 10-7 at halftime to set up the decisive third quarter. Mancuso's 1-yard TD plunge capped a 7-play, 79-yard drive with 6:22 left in the third.
As the quarter drew to a close, Maine regained the lead when Joe Fagnano threw an 87-yard TD pass to Andre Miller. Richmond (3-3, 1-1 Colonial Athletic Association) pinned Maine at its own 1-yard line after a 68-yard punt by Jake Larson. Maine's four-play, 99-yard drive took a little more than two minutes.
Following a short kickoff that Aaron Dykes returned 15 yards to Richmond's 34-yard line, Mancuso connected with Fuller for a 21-17 advantage with 30 seconds left in the third.
Fuller finished 127 yards on three receptions.
Miller tallied 115 yards in receiving yards with four receptions for Maine (2-4, 0-3).