The former Everton man had a great chance to give his side the lead early on, but he could only head his effort straight at Michael Gspurning.
Seattle nearly went ahead just before the break when Eddie Johnson found space inside the penalty area, but his shot was well saved by Luis Robles.
The Sounders made up for that missed opportunity when they took the lead in the 45th minute from the penalty spot after Markus Holgersson handled in the area. Brad Evans made no mistake with the spot kick.
After the break, Johnson had the chance to double his side's advantage but his effort was well saved by Robles, who was forced into an acrobatic stop.
Both sides were denied by the woodwork before Cahill popped up with the equaliser as a loose ball bounced to him in the penalty area and he dispatched it emphatically into the top corner to earn his side a point.