The Union ended 12-12-10 for 46 points. Sporting, which is going for the Supporter's Shield for the most MLS points, now leads with 58. The New York Red Bulls could surpass that total with a win on Sunday over visiting Chicago.
After making the playoffs in their second season, the Union fell to 10-18-6 last year.
Despite the improvement in 2013, this season was going to be judged a success by whether they made the playoffs.
"We let ourselves down and didn't really learn from last year," said Jack McInerney, who scored the Union's lone goal and finished with a team-high 12.
Kansas City took a 1-0 lead when U.S. national team member Graham Zusi headed in a bouncing ball while stationed at the far post in the 47th minute. Matt Bessler started the play with a long throw-in that was headed by Lawrence Olum to Zusi, who headed it in for his sixth goal.
Needing to win, the Union had to gamble and flood the offensive zone. It appeared that Sporting was in a defensive shell, but coach Peter Vermes, the Delran product, said that wasn't the case.
"It was more the way they started play [with an attacking style], and we had to adapt," Vermes said.
The Union had several chances and finally cashed in when McInerney one-timed a header on goal by Parke in the 88th minute.
That goal woke up Kansas City.
Moments after Union keeper Zac MacMath stoned Jacob Peterson in stoppage time, Sporting scored the game-winner by a wide-open Olum on a feed from Teal Bunbury.
Now begins the not-so-long offseason. Training camp begins in late January.
"It is hard for me to sit here and tell you guys that I think the future looks bright [after such a tough loss], but, in fact, I really believe that it does," team manager John Hackworth said.