The sophomore transfer from Hofstra, who had career highs in both categories, scored 15 of his points as the Minutemen (2-0) outscored the Huskies 43-39 in the second half.
The Huskies (1-1) brought it within six, 57-51, with 10:28 to go on Joel Smith’s 3-pointer. But Williams took over, dishing to Freddie Riley for a layup on the break, and then scoring back-to-back layups of his own to make it 65-61 with 7:38 remaining.
The Minutemen then built their lead to as many as 17, 55-38, with 13:38 to play.
Terrell Vinson added 14 points and Sampson Carter 12 for UMass, which outscored the Huskies 28-6 on the fast break.
Jonathan Lee and Joel Smith scored 15 points each for Northeastern.