Sue Bird added 17 points for the Storm (15-17), who clinched their fourth straight playoff berth earlier in the night when fifth-place Houston lost at San Antonio.
Jackson rejoined the team after spending the week in Australia visiting her ailing 91-year-old grandmother, who died Friday following a stroke.
"It has been a hard week emotionally,'' said Jackson, who missed the Storm's blowout losses at Minnesota and Detroit. "I went through it before (in 2004), but it's no easier. I was lucky I got the week (to go home). That's the hardest thing about being on the other side of the world.''
Lennox scored 17 points in the first quarter and combined with Bird to score 26 of Seattle's 28 points.
"I told (Jackson) I would carry it tonight,'' Lennox said. "I just appreciate her coming back and being on the court with us.''
Alana Beard and Monique Currie scored 13 points apiece for Washington (14-16), which snapped a four-game winning streak and had its lead cut to 1 1/2 games over fifth-place New York in the Eastern Conference. Beard, who came in averaging 21.8 points per game, shot just 3-for-14 from the field.
"They're a good team, and with Lauren back, they are a much different team,'' Mystics coach Tree Rollins said. "And we had trouble in the first half with our forwards. (Jackson) is way too much for most of our fives, and was in her zone tonight.''
Jackson scored 16 points in the third quarter - including 11 during Seattle's 18-0 run to open the period. Jackson hit a 3-pointer on the Storm's first shot of the third quarter, Lennox hit a short shot from the right of the basket, and Bird had a 3. Jackson added another 3 as Seattle pulled away.
"They came out in the second half determined and got us back on our heels,'' Currie said. "We couldn't compete with that.''
Washington missed its first 13 shots of the quarter and didn't score until 5:57 remained when Currie hit the second of two free-throw attempts. The Mystics didn't make their first field goal of the third until 3:36 was left, on a turnaround five-footer from the right of the hoop by DeLisha Milton-Jones.
"We had lost our momentum in the third quarter in some games,'' Storm coach Anne Donovan said. "We started the game and started the second half really pushing the tempo. We're at our best when we're in transition.''
Washington, which shot just 6-for-34 in the second and third quarters, trailed 43-38 at halftime.