Williams, who was 8-for-15 from the floor, recorded his sixth straight game with at least 20 points. Witherspoon missed only one of his five shots in the game, including hitting 2 of 3 outside the arc.
Memphis (6-1) never trailed, eventually building the lead to 31.
Branden Johnson led the Bobcats (4-4) with 12 points -- the only Montana State player in double figures. Bobby Howard, Montana State's leading scorer at 15.6 per game, was limited to six points on 3 of 8 from the field.
The Bobcats shot just 34.5 percent for the game (19 of 55) and committed 19 turnovers.