The visitors, without the injured Frank Lampard, looked to make a bright start with David Villa seeing his early shot deflected around Donovan Ricketts's post by the outstretched foot of AJ DeLaGarza.
Galaxy could have taken the lead themselves in the opening stages when Gyasi Zardes pulled a cross back for Robbie Keane on the edge of the area, but the Irishman's effort was wide of Josh Saunders's goal.
Keane then struck the crossbar with a powerful strike after Saunders dropped Sebastian Lletget's corner.
Bruce Arena's men thought they had broken the deadlock on the half-hour mark when DeLaGarza stroked the ball into the net, but was flagged for offside.
Galaxy finally took the lead as Lletget's quickly-taken free kick found Keane in acres of space as the New York defence were slow to react before the captain found Zardes, who headed home from close range.
Keane scored the hosts' second minutes after the restart after racing on to a through-ball from Giovani dos Santos.
The Mexican forward then all but wrapped up victory on 67 minutes with a deft chip over the onrushing Saunders before a further defensive lapse allowed Lletget to get a fourth moments later.
Villa pulled a goal back from the penalty spot with 10 minutes left, but Keane scored his second of the game to cap off an emphatic victory that sees Los Angeles leapfrog Vancouver Whitecaps at the top in the West. body check tags ::