Williamson had 12 points, matched by teammates Jeremi Booth and Donald Sims. Williamson also had 12 rebounds as Appalachian State (8-9, 4-3 Southern Conference) beat the Phoenix (8-10, 2-5) for the fifth straight meeting.
The Mountaineers bolted to an 11-point lead as Anthony Breeze made two free throws for a 20-9 advantage with 11:23 left in the first half.
The Phoenix answered with 10 straight points, pulling to within one, but neither tied the score nor took the lead. They rallied to within one point on a second occasion, but never got over the hump and trailed 37-34 at halftime.