The Galaxy (5-3-2) had lost two games in a row, but scored just 3 minutes into the contest and then dominated the second half to move into a tie with Real Salt Lake for third place in the Western Conference.
Philadelphia goaltender Zach MacMath could not handle a corner kick in the box from Donovan in the early stages. The ball slipped off MacMath's hands and was ruled to be an own goal.
The Union tied it in the 23rd minute on a goal by Amobi Okugo, after which the Galaxy took over.
Robbie Keane, Hector Jimenez and Donovan all scored in the second half. Keane returned to the starting lineup after missing six of the previous games.
Philadelphia (4-4-3) remained in fifth place in the East.
Beyonce flaunts bikini body, Blue Ivy in vacation pics
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'