The expansion franchise accounted for last season’s victors 2-1 at RKF Stadium on Wednesday night thanks to a 67th minute goal from Fredy Montero and a goal four minutes from time from Roger Levesque.
DC managed to get one back in the 89th minute through Clyde Simms but it was not enough as the Sounders marched on to record a famous victory.
The match was marred by an incident after Montero’s goal where United keeper Josh Wicks stomped on the arm of the Colombian and was sent off, leaving the capital-based club in disarray.
Sounders keeper Kasey Keller was ecstatic after the match and was targeting big things for the franchise.
"This is going to go a long way," Seattle goalkeeper Kasey Keller told the Fox Soccer Channel.
"Any time you can pick up silverware in your first year, it's tremendous.
"I don't see no reason why we can't become one of the dominant teams in MLS years to come."
Coach Sigi Schmid was equally as excited, saying: “Obviously you dream about things like that.
"You're not sure it's going to happen. To qualify next year for CONCACAF (Champions League), it's so great for our fans."