RSL (12-9-4, 40 points) extended its unbeaten streak to six matches and moved past Seattle (11-8-6, 39 points) in the Western Conference standings.
RSL also improved to 3-0-1 under interim coach Freddy Juarez, who was named the coach for the remainder of the season after Mike Petke was fired Sunday. Petke was coming off a three-game suspension for directing a homophobic slur to referees two weeks ago during a match against Tigres UANL in the inaugural Leagues Cup.
RSL goalkeeper Nick Rimando, 40, who has announced this will be his last year, didn't face a shot on goal in recording his seventh shutout of the season and the MLS-record 151st of his career.
Johnson opened the scoring in the 25th minute off an assist from Aaron Herrera.
After a giveaway by Seattle's Jordy Delem, Herrera got the ball to Johnson, who was one-on-one with Cristian Roldan going down the right wing. Johnson cut inside at the top of the 18-yard box and unleashed a shot that deflected off Delem's leg and into the left side netting, leaving Sounders goalkeeper Stefan Frei no chance because of the redirection.
The goal was the ninth of the season for Johnson, breaking a tie with Rusnak for the team lead.
But Rusnak pulled even with Johnson in the 71st minute. Baird dribbled the ball down the left wing and passed to Damir Kreilach at the top of the 18-yard box in the middle of the field. Kreilach quickly forwarded the ball to his right to Rusnak, whose one-timer from 18 yards found the lower left corner of the net.
Baird put the game out of reach with a breakaway goal in the 87th minute, assisted by Kreilach.
Frei made one save for the Sounders, whose winless streak reached three matches (0-2-1).