Larry Toomey had 13 points and Kevin Ferguson 12 for the Black Knights (16-14), who lost at Lehigh 81-75 on Saturday after winning five consecutive games, including a 72-58 victory at American on Feb. 23.
Tony Wroblicky had 12 points and eight rebounds and Stephen Lumpkins scored 11 for the Eagles (10-20), who have lost five in a row.
It was American University’s first tournament road game in 12 years in the Patriot League.
American trailed 9-8 on Wroblicky’s layup with 14:05 remaining in the first half, but Army went on a 27-7 run to lead 36-15 on 3:46 Josh Herbeck’s 3-pointer with 3:46 left.
Army’s Kyle Wilson hit a 3-pointer with 6 seconds left to lead 39-20 at halftime.