Donovan first helped out on goals from Jovan Kirovski, Omar Gonzalez and Todd Dunivant then added his own marker to put an exclamation point on the Galaxy's win.
Los Angeles (7-0-1) got all the scoring it would need in the 22nd minute on Kirovski's goal. The Galaxy then put the game away in second half when Gonzalez scored seven minutes in and Dunivant scored in the 57th minute.
Donovan, who raised his MLS-leading assists total to nine, scored his goal in the 67th minute.
Seattle (2-3-3), which lost for the second time in five home matches, fell 12 points behind the Galaxy in the Western Conference.