The Revolution should have taken the lead in the 10th minute when Patrick Mullins fired an effort at goal which was turned into the path of Teal Bunbury six yards out, but Fabinho blocked his goalbound shot on the line.
The Union took the lead in the 42nd minute when Le Toux capitalised on a poor back header from Jose Goncalves and lifted the ball over Knighton into the net.
After the break, the Revolution tried to hit back and Bunbury had another good opportunity inside the box, but he could only steer Andrew Farrell's cross wide of the post.
Philadelphia doubled their lead in spectacular fashion when Danny Cruz picked up the ball inside his own half and ran the length of the field before rifling a shot past Knighton into the bottom corner.
Le Toux sealed the win for the visitors four minutes later though as he drilled a low effort into the bottom corner from close range.