Cal State Fullerton took a 10-point lead into the break before Georgi Funtarov hit a 3-pointer to pull UC Davis within 46-39 with 16:34 to play. Williams answered with five straight points to stretch the lead to 51-39 and the Titans led by double digits the rest of the way.
Both Williams and Harris hit three 3-pointers and made seven free throws.
Cal State Fullerton (8-15, 3-6 Big West) bounced back after having lost three-straight, including a 92-83 overtime loss to Cal State Northridge on Feb. 8.
Corey Hawkins scored 21 points on 7-of-14 shooting and had five rebounds and four assists to lead UC Davis (8-17, 3-7), which has lost three of its last four games.