The Hawks (11-19, 5-11 Atlantic 10 Conference) clinched the final seed in the 12-team conference tournament that begins March 9. LaSalle (12-18, 4-12), did not make the tournament because of its loss and Massachussetts' win over Rhode Island.
Williamson made all 10 of his free throw attempts but was only 4 of 10 from the field. Darrin Govens added 17 points, Carl Jones had 16, and Idris Hilliard had 13 points and 10 rebounds for the Hawks.
St. Joseph's never trailed and led 38-27 at halftime. The Hawks built their biggest lead of the game at 73-57 after a layup by Jones with 1:38 to play. The basket capped a 12-2 run for the Hawks.
Kimmani Barrett scored 15 points and Rodney Green added 12 for the Explorers, who lost 10 of their last 12 games.