Los Angeles took the lead in the 23rd minute when Gyasi Zardes fired a strike at goal from inside the box, only to see his effort crash off the post, but Baggio Husidic was on hand to turn the ball home.
The Galaxy doubled their advantage when Zardes caused problems for the visitors once again and he guided the ball into the path of Juninho, who found the bottom corner of the net from close range.
Philadelphia got a goal back in the 58th minute when CJ Sapong collected the ball on the edge of the area and turned his man before rifling an effort into the bottom corner.
Sebastian Lletget scored the fifth goal for the Galaxy to confirm their commanding victory after he was teed up by Zardes and slotted the ball past Sylvestre.
The Union were denied a late second strike when Cristian Maidana let fly from distance, but his strike crashed off the bar and went over as the Galaxy cruised to a comfortable win.