Despite the visitors looking the more likely side to open the scoring, Rasmus Højlund had other ideas as he drilled home on 23 minutes to give Manchester United the lead.
West Ham really should have drawn level just after half-time with Emerson but he was made to rue that missed chance as Alejandro Garnacho doubled their lead moments later.
Garnacho was at the double to wrap up the three points with Erik ten Hag's side finding their rhythm at the right time of the season while concerns grow for the Hammers who are now winless in their last six.
Story of the match
Manchester United midfield sensation Kobbie Mainoo retained his starting spot after scoring a late winner in Thursday's eventful 4-3 victory over Wolverhampton Wanderers.
Ten Hag's only change saw Raphaël Varane, who recently came back from an injury, go to the bench in place of Harry Maguire.
For the Hammers, Kalvin Phillips, who made his debut in Thursday's 1-1 draw with Bournemouth following his loan move from Manchester City, was replaced by Vladimír Coufal who returned from suspension.
Konstantinos Mavropanos also dropped to the bench with Nayef Aguerd coming in his place at centre-back.
Before the game, tributes were paid to the Manchester United players who died in the Munich Air Disaster on the 66th anniversary as wreaths were laid by both managers.
The East Londoners had started slow in the draw to Bournemouth, conceding in the third minute, but they were focused from the off in this one and had their first chance inside seven minutes as Jarrod Bowen's tame effort warmed the gloves of André Onana.
Minutes later, West Ham were looking to make their early dominance count. James Ward-Prowse's whipped delivery from a corner fell to the head of Tomáš Souček whose header deflected off Edson Álvarez and Onana was on hand to parry away.
The Red Devils responded well as Bruno Fernandes saw his strike from distance tipped away at full stretch by Alphonse Areola.
On 23 minutes, the hosts opened the scoring courtesy of the in-form Højlund. The Norwegian striker got on the end of Casemiro's pass and he got around Aguerd with ease before applying the finish to match as he converted into the corner.
That is now four games in a row that Højlund has scored in, beginning to show why the three-time UEFA Champions League winners forked out £72m in the summer for his services.
West Ham nearly navigated a route back into proceedings on the half-hour mark as Mohammed Kudus failed to make the most of Bowen's cross as the Ghanaian's header was easily gathered by Onana.
It was Manchester United who nearly added to their lead just before the break as Alejandro Garnacho picked out goalscorer Højlund but he failed to find the target.
The visiting side came close to making the ideal start to the second half on 47 minutes. Maguire was dispossessed by Emerson and the left-back drove forward before snatching at his chance and firing wide.
The Italian was made to pay for his missed chance as Manchester United extended their lead moments later. Fernandes' delightful cross-field ball fell to the feet of Garnacho and his strike took a wicked deflection off of Aguerd and Łukasz Fabiański, a half-time substitution for Areola, was helpless in the West Ham goal.
Even though they were trailing by two, West Ham were still in this one but had very little to show for their endeavours. Bowen was sent bearing down on goal but Diogo Dalot intervened with a goal-saving challenge to deny the England international his 12th Premier League goal of the season.
Moyes made two changes, bringing on Phillips and Maxwel Cornet, in pursuit of the next goal of the game but they struggled to test Onana in the closing stages. Phillips, returning to Manchester, summed up West Ham's afternoon with his strike from distance that flew wide of the mark.
The three points were wrapped up five minutes from time as substitute Scott McTominay dispossessed Phillips and the former provided for Garnacho who showed coolness beyond his years to dispatch into the corner.
The Red Devils now close the gap to fifth-placed Tottenham Hotspur to six points while West Ham remain in seventh place.
Player of the match
Rasmus Højlund - Manchester United
Birthday-boy Højlund was once again in fine form with today's 3-0 victory over West Ham.
Despite a slow start in Manchester when he arrived on British shores from Atalanta, the 21-year-old just cannot stop scoring at the moment and is likely to spearhead Manchester United's attack for the foreseeable future.
He sent the Red Devils on their way with his composed strike with 23 minutes on the clock, sending Aguerd out of the equation and finding the corner with ease.
With the main aim of securing a top-four finish, Højlund could be an influential figure in the remaining months of the campaign.
Garnacho was also on fine form this afternoon and is cementing his place on the right-hand side of Manchester United's attack.
