The U.S. national team captain made his debut as the Sounders grabbed three vital points north of the border.
The Sounders took an early lead when Brad Evans found Eddie Johnson making a run into the back post. Johnson headed back across goal to Mauro Rosales, who was free to smash the ball home with his left foot.
Dempsey, who started on the bench, made his debut earlier than expected. An injury to Obafemi Martins saw the forward forced from action and Dempsey entered as his replacement in the 34th minute.
Dempsey nearly got on the board with his first touch of the game just minutes later, as he, Johnson and Rosales broke forward against two Toronto defenders, with Johnson laying the ball off for Dempsey just outside the box. A stepover seemed to have opened up space for the shot, but Ashtone Morgan did very well to recover and kill off the chance.
The Sounders did double the lead before the break though. Lamar Neagle's dash down the left and cross into the box took an odd deflection off of a Toronto defender and drifted into the back post.
Jonathan Osorio made sure the second half would be interesting. Just 30 seconds after the restart he had Toronto on the board, cutting back across Osvaldo Alonso and clipping his shot over the sliding Marcus Hahnemann and in to make it 2-1.
Robert Earnshaw, who came into the match early in the second half for Toronto, nearly had the score level in the 60th with a left-footed shot that beat Hahnemann but came back off the post and was cleared by Seattle.
Dempsey's best chance at a goal came midway through the second half and was of his own making. A quick turn beat two defenders, and a second change of direction beat a third before he unleashed a left-footed shot that looked destined for the corner until Joe Bendik made a fine sprawling save to keep the goal out.
Toronto would see limited chances to get the score level after that point, with Seattle settling in and taking a bit more control of the match, but a stoppage-time save from Hahnemann on Jeremy Brockie was still needed to preserve the win.