The Blues were the better team early on but the game changed in the 17th minute when Kurt Zouma brought down Marcus Rashford inside the box.
The England striker dusted himself down and confidently dispatched his penalty past Kepa Arrizabalaga.
Anthony Martial then scored United’s second goal in the 65th minute before Rashford sent United fans into dreamland by netting his second of the afternoon minutes later. Daniel James then added gloss to the scoreline with a fourth after coming off the bench.
This was a dream start to the season for United, whose fans will be hugely encouraged for the season ahead.
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Lampard sprang a couple of surprises with his starting line-up. Christian Pulisic was named among the substitutes, as was N’Golo Kante.
Ole Gunnar Solskjaer, meanwhile, deployed Harry Maguire and Aaron Wan-Bissaka from the start. Fellow summer signing James started on the bench.
Tammy Abraham and Mason Mount were both in Chelsea’s line-up and the former went close to breaking the deadlock inside the opening five minutes with a brilliant strike from distance.
Had that found the back of the net, rather than the far post, it may have been a very different story.
However, it was Rashford who scored the opening goal from the penalty spot after Zouma’s mistimed tackle inside the box.
Zouma, who spent last season on loan at Everton, endured a difficult afternoon at Old Trafford.
United grew in confidence after the goal and began to dominate possession while moving the ball quicker.
Wan-Bissaka was the star performer during the opening 45 minutes. The right-back looked fantastic on his competitive debut and fans were soon chanting his name.
But Chelsea had their chances to equalise before the break. Ross Barkley went close on two occasions before Emerson hit the crossbar from close range.
Lampard cut a frustrated figure on the touchline. He must have been wondering how his team weren’t at least level with the hosts.
Emerson forced De Gea into another save early in the second half, but Chelsea were killed off thanks to two quick-fire goals from Martial and Rashford.
Maguire, who didn’t put a foot wrong on his debut, started the move which eventually led to Martial doubling United’s advantage.
Minutes later it was 3-0 when Paul Pogba picked out Rashford, who made no mistake after finding himself one-on-one with Kepa.
And James made it an afternoon that United fans will remember for a long time by coming off the bench to score his first competitive goal for the Red Devils after brilliant work from Pogba, who was magnificent.
United fans cruelly chanted 'You're getting sacked in the morning' towards Lampard.
And the new Chelsea boss has plenty to think about after this humiliating opening-weekend result.News Now - Sport News