Clint Dempsey made his debut as a first-half substitute after sealing a return to the USA from Tottenham Hotspur.
The visitors started well, putting Toronto under pressure with a series of early free kicks.
Former Newcastle United striker Obafemi Martins then missed with a couple of long-range efforts, before going off injured and being replaced by Dempsey.
Dempsey was immediately involved, winning the free kick that led to Seattle doubling their lead when Doneil Henry headed past his own goalkeeper.
Osorio missed a chance to equalise shortly after before Robert Earnshaw struck the post from the edge of the box for Toronto.
Dempsey had a late chance to make it 3-1 but Seattle were able to hang on for all three points regardless.