Chicago used an own goal in the first half and a Mike Magee strike late to take three points.
A Fire free kick deflected off of United defender Nick Deleon and drifted into the net for the game's opening goal.
Dwayne De Rosario nearly pulled one back almost immediately, but his hit from outside the box rattled off the crossbar and came back out.
Chicago waited until late on to put the game on ice. Mike Magee had a chance in the 83rd, but couldn't finish off a breakaway.
Patrick Nyarko picked out Magee two minutes later and the former LA Galaxy star wasn't going to miss a second glorious chance, putting the game to bed by putting his shot in the bottom corner.