Chivas played the final 16 minutes with 10 players after Marco Delgado was ejected in the 74th minute.
Federal Way's Lamar Neagle scored in the 24th minute for Seattle (4-2-1), which is 3-0-1 on the road this season.
Erick Torres opened the scoring in the fifth minute on a penalty kick for Chivas (1-3-3).
Seattle responded in the 24th minute when Chivas goalkeeper Dan Kennedy could not hold onto the ball and Neagle sent the rebound into an empty net.
Martins and Dempsey combined on the game-winner. Martins fed Dempsey, who dribbled toward the area and sent the ball back to Martins. As Kennedy ran off his line, Martins chipped the ball into the back of the net.
After three straight road games, the Sounders return home for three straight. That stretch starts with a match against Colorado on Saturday, April 26th.