Luke Martinez added 12 points for the Cowboys (16-3, 2-1 Mountain West), who never trailed.
Greg Smith led Colorado State (13-5, 2-1) with 18 points. Jesse Carr scored 12 and Pierce Hornung had 10 points. The Cowboys held Wes Eikmeier, who was averaging 16.7 points per game, to two points.
Washington banked in a 3-pointer as the shot clock expired to give Wyoming a 64-49 lead with 2:56 to play, then repeated the trick with 42 seconds left to make it 70-49.
Colorado State entered as the nation’s fourth-best 3-point shooting team at 44 percent, but connected on 29 percent from behind the arc on Saturday.