Thompson had a career-high 17 points, while matching a high of 12 rebounds for his second double-double. Castellanos also had career highs of 16 points and nine assists for the Mountaineers (1-1).
Raven Barber added 13 points for Mount St. Mary's, which shot 50 percent from the floor (21 of 42), while holding Hartford to just 27.7 percent shooting (13 of 47).
Hartford (0-3) outscored the Mountaineers 25-3 off the bench, but was outrebounded 38-26 and struggled from the free-throw line.
The Hawks were just 20 of 31 from the foul line (64.5 percent).
The shooting woes were nothing new for Hartford, which was averaging 33.6 percent from the field coming in, scoring 52.5 points per game.