United have found goals hard to come by at home this season and it looked like being another frustrating night when they went in level at the break having dominated the first half.
However, an inspired second-half performance saw the Red Devils reached a 14th League Cup semi-final as they put the Hammers to the sword without boss Jose Mourinho on the touchline as he served as one-game ban..
It had started well for United with a goal after just two minutes from Zlatan Ibrahimovic following a flowing move. The Swede was able to dink the ball over the on rushing Adrian after a wonderful back heel from Henrik Mkhitaryan into his path.
It should have been two for United five minutes later when Ibrahimovic was denied twice in quick succession as the home side threatened to rack up a big score early on.
United had further chances, most notably through Anthony Martial, but the opportunities did start to dry up as the half wore on and West Ham grew into the game.
Despite improving, it was still a surprise when the Hammers levelled in the 38th minute, David De Gea failing to hold Dimitri Payet’s effort and giving former United striker Ashley Fletcher an easy tap in to open his account for the Londoners.
Fletcher’s goal was to be the highlight of the night for the Hammers though as United came out with the bit between their teeth in the second half, going back ahead three minutes after the restart through Martial.
Another excellent back heel set the home side on their way, Antonio Valencia fooling the Hammers’ defence and finding man of the match Mkhitaryan, who fed Martial, the Frenchman slamming it into the roof of the net.
Adrian then kept the Hammers in it but his resistance was finally broken just after the hour mark when Martial got his second, tapping home from Valencia’s low cross to finish off a great team move.
Despite three excellent goals to to enjoy, the loudest cheer of the night was reserved for Bastian Schweinsteiger when he came on as a substitute in the final few minutes for his first appearance of the season.
There was just time left for Ibrahimovic to get his second and United’s fourth, tapping home after some good work by Ander Herrera in the build-up.