The hosts started brightly as Obafemi Martins scored after just six minutes following a good through ball from Dempsey.
The 32-year-old American nearly doubled his side's lead four minutes later but his left-footed effort was saved by Chris Konopka.
Michael Bradley and Jonathan Osorio both had opportunities to equalise before half time but they failed to find the back of the net. Martins nearly grabbed his second before the break but his shot was comfortably saved by Konopka.
Toronto pushed forward in search of an equaliser in the second half and they were rewarded when Eriq Zavaleta scored with a header from a corner.
Seattle tried to respond quickly from this setback as both Martins and Dempsey tested Konopka with shots from close range. After a period of sustained pressure, Dempsey finally got his goal when his shot from inside the box found its way past the keeper 13 minutes from time.