With Plattsburgh up 67-64, Williams nailed a pair of free throws with 10.5 seconds left and then Jerrad Dumont stole the ensuing inbounds pass to seal the win. Williams was 8 of 13 from the field, including 4 of 5 from the 3-point stripe, and recorded four assists and two steals.
The Saints had a chance to tie the game on their previous possession, but Tom Wopperer's 3-point attempt from the top of the key glanced off the front of the rim. Wopperer led his team with 22 points.
Plattsburgh built a comfortable 41-25 halftime lead on 17 of 32 shooting for 53.1 percent before having to hold off a scrappy Saints squad, who outscored the Cardinals 39-28 in the second half.
Travis Gorham grabbed a game-best 15 rebounds, scored nine points and had four steals for Plattsburgh. Kris Gibbs-Smith added 14 points.
Bela Vonnak contributed 16 points on 8 of 11 shooting for the Saints.
Plattsburgh will return to action this weekend against State University of New York Athletic Conference foe Cortland State (Friday, 8 p.m.) and non-league Ithaca (Saturday, 2 p.m.), both at Memorial Hall.