Soon after Donovan went from provider to scorer, collecting a pass from Gyasi Zardes before finishing confidently.
Zardes rifled home in a third just three minutes later, blasting into the corner with an emphatic left-footed strike.
Just before the break Donovan got his second to effectively end the match as a contest, with a clever chip allowing the winger to move level with Jeff Cunningham as Major League Soccer's all-time top scorer on 135 goals.
Chivas improved significantly after the break but were unable to find a way back into the match, with Galaxy closing out a morale-boosting home win.
There was still time for Keane to add his second and the Galaxy's fifth in the closing stages, beating Dan Kennedy with a precise half-volley after controlling the ball with his chest.