Keane broke a scoreless tie late in the first half off a perfect pass from Landon Donovan and then assisted on Mike Magee's right-footed shot at the end of a fast break in the 67th minute.
The Irishman handed the Galaxy a formidable three-goal advantage 3 minutes later, directing Christian Wilhelmsson's attempt into the net.
The Sounders will now need to make up the difference in the second leg of their first-ever appearance ever in an MLS conference final.
That contest is set for Nov. 18 at CenturyLink Field.
Astronomers offer more expansive view of universe
LGBT community has 'bullied the American people': Bachmann