Teal Bunbury got Kansas off to the perfect start on 14 minutes, scoring his fifth goal of the season with a header from close range.
DC United responded well and were back on terms just 10 minutes later through Nick DeLeon.
Both sides had chances to gain control in the second half but it was Kansas City who scored the winning goal through Graham Zusi.
After striking the woodwork twice Zusi finally found the back of the net on 63 minutes, finishing neatly through DC keeper Bill Hamid's legs.
The result gave Sporting their first home win since June 16 and moved them back ahead of New York Red Bulls in the battle to win Major League Soccer's Eastern Conference.