Lumpkins had 11 points and 10 rebounds for the Eagles (6-3), who started the season 5-0.
The Retrievers (0-10) have lost one more game than they did to open the 2009-10 season, when they beat American 59-57 before starting an 11-game losing streak.
UMBC took an early seven-point lead, but the Eagles used a 13-2 run to lead 16-2. American took a 35-16 halftime advantage.
In the second half, American led by as many as 23, although the Retrievers did get within nine points with 1:28 to go.
Vlad Moldoveanu, who came in averaging 20.3 points per game for the Eagles, was held to nine points on 2-for-10 shooting.