Seattle (23-7) claimed the first two matchups with Washington, winning 81-77 on May 29 and 97-91 on July 8, both of which were in Seattle. The Mystics have won two straight at home against the Storm.
The Storm are coming off a 94-79 win over the Indiana Fever on Tuesday. With the victory, Seattle secured its best road record in franchise history (12-3).
Natasha Howard led the way with 19 points, eight rebounds and a career-high-tying four assists, while Breanna Stewart posted her seventh double-double of the season with 17 points and 12 rebounds.
Sue Bird logged 22 minutes to become the league's all-time leader in career minutes, passing Tina Thompson (16,088).
"I thought the ball movement was key, our ability for us to get strong bench play on a back-to-back, and Natasha Howard's movement within our offense was just excellent," coach Dan Hughes said.
With the win, Seattle kept its 3 1/2-game lead over second-place Atlanta (19-10).
The Mystics secured a playoff spot with a 103-98 road win at Phoenix on Tuesday night.
Elena Delle Donne scored 30 points on 10-of-14 shooting, Kristi Toliver added 25 and LaToya Sanders scored 19 for Washington (18-11), which can still finish in second and earn byes to the semifinals.
"We clinched a playoff spot for us tonight, clinched a winning road record for the year," coach Mike Thibault said. "That is pretty rewarding thing considering all the stuff we went through earlier in the year."
The Mystics made 10 of their first 12 shots to take an early nine-point lead and led by 10 with 2:17 remaining before the Mercury rallied to within 100-98. Toliver made 1 of 2 free throws with 16.2 seconds left and, after a Phoenix turnover, she hit two more to seal the win.
"Everything is feeling really good, we're spacing the floor really well, trusting in the extra pass finding one another," Delle Donne said. "Defensively when we need to get stops we are able to come away with some. Huge road victory, now it is time to go home and get ready for Seattle."
Toliver and Natasha Cloud each hit four 3-pointers and the Mystics made 11 of 23 as a team. Washington shot 56.1 percent (37 of 66) from the field.
The Storm lead the WNBA in field goal percentage and 3-point percentage while the Mystics are sixth in each category. Seattle also leads the league in assists and steals.