Patrice Bernier assisted on three Impact goals to help Montreal (4-7-3) end a two-game losing streak.
Eddie Johnson scored for the Sounders (7-4-3), who were reduced to 10 men in the 57th minute when Jhon Kennedy Hurtado was sent off for a nasty tackle on Collen Warner.
Seattle is winless in its last five (0-3-2).
Bernier made a quick move at the right side of the area and slipped the ball to Martin at the top of the box to curl a shot inside the far post in the 18th minute.
Mapp scored 6 minutes into the second half when he took a long pass from Bernier and cut most of the way across the front of the penalty area before firing a low shot against the flow inside the left post.
Only seven minutes later, Bernier picked off a Seattle clearing attempt and slipped a pass to Wenger to score from the right side.
The Sounders got one back when Fredy Montero sent Johnson in alone to chip a ball over Ricketts in the 62nd minute.
Neagle, a former Sounder, replaced Mapp in the 62nd minute and scored his first goal of the year.
The game was the opener of the revamped and expanded Saputo Stadium. The expansion Impact played their first five MLS games next door at Olympic Stadium as work was completed on their grass field home park.