Donovan's 46th-minute goal gave Los Angeles its third straight win and gave him his league-leading eighth goal of the season.
The Galaxy played without David Beckham, who was given leave to play with former team Manchester United in a one-off friendly against Juventus to mark the end of best friend Gary Neville's career.
In other MLS matches, FC Dallas beat the Seattle Sounders 1-0, while the Colorado Rapids and New York Red Bulls played out a 2-2 draw.
Brek Shea scored in the 18th minute to give Seattle the home win over Dallas.
Andrew Jacobson sent a quick, short pass ahead to Shea just over the midfield line, where he sped downfield toward the penalty area. With Sounders goalkeeper Kasey Keller coming off his line, Shea fired from the top of the box into the back right corner for his fourth goal of the season.
Luke Rodgers scored in the 33rd minute to cap a six-minute scoring flurry that lifted New York to a draw with Colorado at Harrison, New Jersey.
Jeff Larentowicz scored twice for the defending MLS champion Rapids, who earned their fourth straight draw.
Larentowicz opened the scoring by tapping in the rebound of his own failed penalty in the 27th minute. That started the league's fastest scoring binge since March 28, 2008, when Houston and San Jose also combined for four goals in six minutes.
Thierry Henry also scored for New York, which moved into a share of the Eastern Conference lead with Philadelphia.
