Troy Brewer added 13 points and for the Eagles (19-10, 10-4 Patriot League), who clinched the third seed in the upcoming conference tournament. American will face sixth-seeded Army in the quarterfinals matchup on Wednesday.
Ryan Willen scored 16 points and grabbed a career-high 13 rebounds for the Leopards (12-17, 7-7), who will be seeded fifth and travel to fourth-seeded Holy Cross for their quarterfinal.
Rob Delaney added 14 points for Lafayette, which has won just one of its last 17 games against American—a 73-71 double-overtime win in the conference semifinals last year.
American led by as many as 20 points but allowed Lafayette to get back into the game late with poor foul shooting.
In one stretch, the Eagles missed four of six foul shots.