Washington (21-8) built a 31-14 advantage by the end of the first quarter and led by double digits the rest of the way.
Kia Nurse scored 24 points, making 6 of 10 from 3-point range for the Liberty (9-20). Bria Hartley and Tina Charles added 16 points apiece.
The Mystics' LaToya Sanders blocked three shots to pass 200 career blocked shots, finishing the game with 201. She finished with eight points.