Kansas City (3-5-3) won for the first time since beating Colorado 1-0 at home on April 10.
Philadelphia, which played a man down from the late first half on, dropped to 2-7-1 heading into the World Cup break of its expansion season.
Kamara scored in the ninth minute and Zusi picked up his first Major League Soccer goal in the 35th, each time after staying onside and beating the left side of the Union's back line.
Kamara took a feed from Davy Arnaud and put a rocket into the upper left corner, and Zusi went to the lower left corner after Ryan Smith found him in space just inside the penalty area.
Union defender Michael Orozco was sent off in the 42nd minute, after drawing his second yellow card for a reckless tackle on Kansas City's Josh Wolff.
Wizards keeper Jimmy Nielsen made two saves and earned his fifth shutout of the season.
(Copyright 2010 Philadelphia Union. Reprinted by permission)
