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.
Celebrity Couples of 2014 [PHOTOS]