It was the first time this season that the Sounders (13-6-8, 47 points) won after allowing the game's first goal.
Alex Caskey started the play that led to the game-winner with a short pass to Fredy Montero on the attacking left side. Montero, going stride for stride with Shalrie Joseph of Chivas, got a step in front of him and crossed the ball through the penalty area.
Johnson met it 8 yards in front of the net and headed it hard off the hands of goalkeeper Dan Kennedy and into the back right corner for his 13th of the season.
Nick LaBrocca opened the scoring for Chivas (7-12-7, 28 points) in the eighth minute. He took a short through ball from Joseph, turned from 16 yards in front, and beat Sounders goalkeeper Michael Gspurning to the back right corner for his second of the year.
Johnson answered in the 28th. With a free kick 10 yards up from the goal line on the far left side of the field, Christian Tiffert floated the ball toward the far post. Johnson, from the top right corner of the 6-yard box, sent a header down to the turf, and it bounced over Kennedy's outstretched arms and into the upper right corner.