The Broncs (8-6, 2-1 Mid-Atlantic) made five of six free throws while forcing the Red Foxes (1-12, 0-3) into four misses and a turnover during the final minute.
Philllip Lawrence hit a 3-pointer to pull Marist within 65-57 with 1:19 to play. The Red Foxes didn't score again until a Lawrence dunk as time ran out.
Sadler scored at the halftime buzzer to tie the game at 29. Marist took a 48-45 lead midway through the second half when Shawn Valentine and Anthony D'Orazio made consecutive jumpers to put Rider up for good. That stretched to an 11-0 run, capped by Valentine's 3-pointer for a 57-48 lead with 5:40 to go.
Chavaughn Lewis led Marist with 20 points, hitting four 3-pointers. The Foxes shot 42 percent, hitting 9 of 25 3s to stay close.