Castellanos shot 3 of 11 from the field but hit eight free throws in the win. Thompson, who had seven rebounds in the second half alone, helped the Mountaineers (3-10) outrebound American 41-24. The rebounding advantage allowed Mount St. Mary's to outscore American 28-18 in the paint and 16-2 on second-chance points.
American (8-7) led 40-38 before the Mountaineers used an 11-2 run over the final 4 minutes to secure the victory. Castellanos knocked down four free throws during the run.
The Eagles led 21-18 at halftime but shot just 30.4 percent (7 of 23) in the second half compared to 45.8 percent (11 of 24) for Mount St. Mary's.
Troy Brewer scored 15 points for American but the Eagles bench only scored four points.