Iona hit 54 percent from the floor, including 13 of 27 from beyond the arc. Leading scorer Sean Armand was held to just 11 points.
Siena (7-12, 3-5 Metro Atlantic) trailed by six points midway through the second half when A.J. English's layup kicked-off an 11-2 Iona run capped by a Williams layup giving the Gaels a 70-57 lead with seven minutes to play.
Tavon Sledge's basket inside the paint extended Iona's lead to 17 with 4:40 left, marking the largest lead of the game. The Gaels coasted to the buzzer, winning by 14 points.
Iona (9-8, 6-2) also beat Siena last Sunday 87-78, and has won seven of its past eight meetings.