Candice Dupree poured in 27 points while 79 victory against the Mystics at Verizon Center.
Nakia Sanford had a team-high nine rebounds and added six points for Phoenix, which snapped a two-game slide.
Crystal Langhorne finished with 27 points and 12 boards but was unable to steer Washington away from its eighth consecutive loss. Matee Ajavon had 15 points, and Jasmine Thomas added 14 off the bench in the setback.
The Mercury led by as many as 22 and were still up by double digits with three minutes left, when Alexis Gray-Lawson hit a jumper for an 83-67 advantage.
The Mystics drastically cut down the lead with layups from DeMya Walker, Langhorne and Kerri Gardin, and a three-pointer from Thomas with 1:20 left pulled them within 83-76.
But Washington missed its next two shots, and Marissa Coleman missed two free throws moments after as Phoenix held on.