Keane broke a scoreless tie just before halftime when Todd Dunivant nodded a pass to the unguarded striker, who easily finished from inside the 6-yard box.
The Irishman made it 2-0 in the 69th minute after Landon Donovan stole the ball in the attacking zone and gave it to Keane, who hammered a left-footed shot inside the far post.
Marcelo Sarvas also scored for the defending MLS champion Galaxy (1-1-0).
United's Nick DeLeon tallied in the 87th minute to spoil the shutout bid of Los Angeles goalkeeper Josh Saunders.
D.C. fell to 0-2-0 with the setback.