Goals from Eric Bailly, Romelu Lukaku, Paul Pogba and Anthony Martial gave José Mourinho's side another emphatic victory to kickstart the season as the top-flight's pace-setters.
The beginning of the first period was quite eventful with both Jordan Ayew and Phil Jones hitting the crossbar inside the first 10 minutes. Marcus Rashford had a great chance before Bailly gave the away side the lead on the stroke of half-time.
Paul Clement's side improved in the second period with chances for Tammy Abraham but three goals in four minutes from Lukaku, Pogba and Martial secured United's second straight 4-0 victory.
Both teams start off brightly
The Red Devils headed to South Wales top of the table and looking to continue their fantastic form from the the previous week, but both sides made a good start to the game.
The Swans clattered the woodwork inside the first three minutes when Jordan Ayew whipped a ball into the mixer which caught David De Gea out, but the 'keeper was saved by the crossbar.
It was then the turn of United to come forward and clatter the bar. Juan Mata curled a free-kick into the area which found the head of Jones, but his effort came back off the woodwork.
The half went quiet as it came towards its end, but Rashford was given a golden opportunity in the 36th minute.
An awful header from Alfie Mawson put Rashford clean through on goal, however the teenager decided to try a chip which went straight into the arms of Łukasz Fabiański.
Bailly in the right place at the right time
It looked like both sides would be heading into the break goalless, however United managed to put themselves ahead on the cusp of the half-time.
A corner-kick found the head of Pogba, who produced an excellent save from Fabiański to tip it on to the crossbar. But Bailly was alive to the situation as he ran in to poke the ball home before the Swans could clear.
Clement's side came out for the second period looking brighter. They dominated the early proceedings of the half and Abraham came close just after the hour mark.
Tom Carroll did well to cross the ball into the box which caught Jones and Bailly out. Abraham had plenty of space but couldn't direct his header on target from a good position.
United quick and deadly in attack
The Red Devils led 1-0 as the clash entered the final ten minutes, but a stellar and devastating four minutes secured another excellent result for Mourinho's men.
They added their second in the 80th minute as Henrikh Mkhitaryan powered United forward. The Armenian played the ball through to Lukaku and he coolly slotted it into the bottom corner.
Once again, the magic of Mkhitaryan created another goal in the 83rd minute. The playmaker played a through ball to Pogba and the Frenchman did brilliantly to chip it over the advancing Fabiański.
The result was sealed in the 84th minute as United added their third goal in four minutes with a devastating attack as Pogba put a ball through to Martial and the Frenchman slotted into the bottom corner as the Red Devils sealed another convincing three points.