Marcus Hammond added 21 points for the Purple Eagles.
Thomasson hit 11 of 14 shots.
Jordan Cintron had 13 points and nine rebounds for Niagara (14-15, 9-11 Metro Atlantic Athletic Conference), which earned its fourth straight home victory.
Jao Ituka had 19 points for the Red Foxes (14-15, 9-11). Ricardo Wright added seven rebounds.
The Purple Eagles evened the season series against the Red Foxes. Marist defeated Niagara 77-70 on Feb. 14.
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