Robbie Keane scored once and had an assist, while Hector Jimenez also scored for the two-time defending MLS Cup champion Galaxy (5-3-2), who snapped a rare two-game losing streak with three second-half goals.
In the process, Donovan became the fourth player in franchise history to play in 200 games. He has 94 career goals, the most in club history, as well as 82 career assists, second most in Galaxy history behind Cobi Jones' 91.
Amobi Okugo scored for the Union, who fell to 4-4-3.
The Galaxy went ahead 1-0 in the third minute on an own goal. A corner kick bounced off Philadelphia goalkeeper Zac MacMath and into the net.
Okugo tied it in the 23rd minute when he headed home a long throw-in by Sheanon Williams.
In the 71st minute, Keane gave the Galaxy a 2-1 advantage. Jimenez sealed the win in the 80th minute and Donovan also scored in the 87th minute.
