Johnson nearly scored another in the 39th, but Los Angeles goalkeeper Jaime Penedo leaped to grab his open shot in the box.
Juninho tallied with the final touch of the first half, bending a free kick from just above the semicircle inside the left post.
Rookie Gyasi Zardes forced a leaping save from Seattle goalkeeper Michael Gspurning with a blast toward the upper-right corner in the 19th minute, and he appeared to put the Galaxy ahead following a free kick 15 minutes into the second half, after Gonzalez headed Juninho's cross into the goalmouth. Referee Silviu Petrescu whistled the Galaxy for a foul, apparently on defender Todd Dunivant, while the ball was in the air.
"I think they had a little more possession than we had, for sure," Seattle coach Sigi Schmid said. "But I think that we had some very dangerous chances. ... I think we had opportunities to get our second goal, but that's the way it is. It's an end-to-end game, an exciting game for the fans, but it wasn't always the best soccer."