The victory gave Louisiana-Lafayette coach Bob Martin his 300th career win at the Division I level.
Williams made 6 of 8 field goal attempts for 15 points. Washington, who was 5 of 6 from the floor in the first half, finished with 14 points, Kasey Shepherd added 13 and Shawn Long and Devonta Walker had 10 apiece for the Ragin' Cajuns (9-5, 3-0 Sun Belt).
Louisiana-Lafayette led 38-24 at the break. Walker started the second half with a layup and followed with a three-point play that started an 18-6 run that led to a 62-42 advantage with 10:48 remaining. The Mountaineers (4-7, 1-1) never threatened.