After converting one spot kick in regulation in Wednesday night's final, Sporting outlasted Seattle in a penalty shootout to derail the Sounders' bid to become the first team to win four straight titles.
"I went out there before the first kick and I told the guys, 'Look, different things are going to happen through these five - maybe up to nine - shots," manager Peter Vermes said after Kansas City won its first Cup since 2004.
"Whatever happens, the next guy up needs to clear his mind and get on with his business."
That's just what Sporting did, finishing with a 3-2 advantage in an up-and-down exchange that had Seattle miss its last three tries and Sporting's Paulo Nagamura make good on his second chance after the first one was saved.
After the match went through overtime tied 1-1, Sporting opened the shootout with Kei Kamara's successful conversion and Brad Evans answered for Seattle.
The Sounders led 2-1 after the second round, when Michael Gspurning saved Roger Espinoza's low shot and Marc Burch beat Jimmy Nielsen with a shot to the right side of the goal.
That was Seattle's last successful attempt.
In the third round, Sporting's Matt Besler scored off the underside of the crossbar and Osvaldo Alonso shot well over the bar.
Kansas City's Graham Zusi chipped high to open the fourth round, but Nielsen saved Christian Tiffert's shot to keep things even and set up the final-round drama.
Gspurning saved Nagamura's shot, but Nagamura was given a second chance when Gspurning was whistled for coming off his line early.
Nagamura made his second attempt in the bottom left corner, and Eddie Johnson sent his answering attempt high.
That sealed Kansas City's first Cup title since the then-Wizards won it in 2004 - also the last time the final went beyond regulation. Kansas City also snapped a seven-game winless streak against Seattle across all competitions.
Kansas City also earned a berth in the 2013-14 CONCACAF Champions League.
Seattle manager Sigi Schmid was unhappy with the retake.
"You can always call a goalkeeper for moving," Schmid said. "Every goalkeeper. Nielsen moved ahead of every shot as well, but he didn't call anything back there. All of a sudden, he calls that indiscriminately."
Nagamura, who sustained a cut on his left cheek early in the match and was bleeding through his bandage when he took the game-winner, said he knew he'd have a second chance as soon as he took the first one.
"If he wasn't off his line, he never could have saved my PK," Nagamura said. "The second one, I was pretty confident. I just put it in and then waited for Johnson to miss it."
Kamara opened the scoring for Kansas City on a penalty kick in the 84th minute after Zach Scott was called for a handball in the area. Scott redeemed himself in the 86th with a header from 12 yards off Mauro Rosales' free kick.
Sporting went up a man in the 119th minute, when Patrick Ianni was given his second yellow card for a foul on C.J. Sapong, but was unable to take advantage.