Bruce Arena's men were forced to do things the hard way as a determined Revolution side forced extra-time at the Stubhub Center, but Keane's 111th-minute strike clinched a record fifth MLS title for Galaxy in Landon Donovan's final match before retiring.
The Los Angeles club were in charge for much of the first half, crafting several good opportunities as they sensed victory was within their grasp.
In fact, Arena's side almost took the lead with barely a minute played, as Robbie Rogers cut in from the left and poked goalwards, only to see Chris Tierney track back well and clear his partially blocked effort off the line.
Keane was the next to go close for Galaxy, as the league MVP exploited space in the Revolution defence, before firing straight at goalkeeper Bobby Shuttleworth.
Revolution held firm until the break, though the deadlock was eventually broken in the 52nd minute, as Gyasi Zardes pounced on Stefan Ishizaki's deflected cross, held off a defender and then stabbed home.
Chances continued to fall for Galaxy, though it was Revolution who ultimately drew level, netting with 11 minutes left of normal time, as Tierney latched on to Patrick Mullins' cut-back and fired through Galaxy goalkeeper Jaime Penedo's legs.
The New England club's efforts went unrewarded, however, as Keane raced on to Marcelo Sarvas' throughball and confidently swept past Shuttleworth in the 111th minute, inflicting a fifth MLS Final defeat on the Revolution and giving strike partner Donovan the perfect send-off.
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