Harewood carried on from his impressive hat-trick against Aston Villa on Monday to score the first and see his shot rebound off Tony Warner for the crucial second.
Luis Boa Morte's late strike was not enough for the home side and West Ham returned back across London with another crucial victory.
Fulham could have opened the scoring after two minutes when Tomasz Radzinski's ball picked out Claus Jensen inside the box.
The Danish international saw his shot superbly beaten out by Roy Carroll's outstretched leg and away to safety.
The home side continued to dominate the opening exchanges and went close again to breaking the deadlock on 13 minutes.
Boa Morte's teasing cross picked out Brian McBride whose close-range header produced another fine save from Carroll diving away to his left.
West Ham forced their way back into the game and created their first major opening on 23 minutes. Harewood picked up the ball inside the box and made a clever turn, but dragged his shot wide of the post.
His strike partner Bobby Zamora then got in on the act 60 seconds later when the ball fell kindly to him and he fired a fierce drive straight into the grasp of Warner in the Fulham goal.
The Hammers had the ball in the back of the net on 27 minutes but Zamora's effort was ruled out for offside. Matthew Etherington's shot was only parried by Warner - the ball fell to Zamora who knocked the ball into an empty net, but the linesman had his flag raised against the former Brighton and Spurs forward.
Carroll came to the rescue of West Ham once again on the half-hour mark when McBride's dipping shot from the edge of the box was turned over the bar by Carroll.
Harewood was denied by the Fulham woodwork on 32 minutes. Yossi Benayoun's long throw-in was flicked on by Zamora into the path of Harewood whose volley came back off the crossbar. The rebound fell straight to Nigel Reo-Coker whose header flew just past the post.
Boa Morte went close again for Fulham on 41 minutes when he made a clever turn on the edge of the area and drilled a low curling shot which edged inches past the post.
Warner almost cost his side on the stroke of half-time when he failed to hold onto a cross - Benayoun picked up the loose ball and crossed back into the area but Fulham defender Carlos Bocanegra was on hand to clear the ball off the line.
West Ham changed the course of the game scoring two goals inside the opening six minutes of the second half. The first came on 46 minutes when Tomas Repka's high ball was flicked on by Zamora into the path of Harewood.
Harewood flicked the ball over Zat Knight and lifted the ball over Warner into the corner of the net.
West Ham made it two on 51 minutes when Harewood was involved in the heart of the action once again.
His close-range shot came back off the post but the ball struck the back of Warner in the Fulham goal and rebounded into the corner of the net.
The Cottagers forced their way back into the game on 67 minutes when Boa Morte's curling shot from the edge of the box sailed past Carroll into the corner of the net.
But West Ham held on to secure another impressive three points to maintain their good start to the season back in the top flight.
MAN OF THE MATCH: Marlon Harewood (West Ham United) - The former Nottingham Forest striker continued his rich vein of form with another outstanding display and fourth goal in a week as the Hammers maintained their excellent start to the season.