The Blues, who sacked Mourinho for a second time last year, led after just 30 seconds when Pedro capitalised on slack defending to roll in.
Gary Cahill smashed in the second after United allowed Eden Hazard's corner to bounce in their box.
United offered little sign of making a comeback, falling further behind when Hazard drilled in a precise 15-yard strike.
N'Golo Kante skipped around a static defence to slot in and seal victory as Chelsea moved within a point of Premier League leaders Manchester City.
The comprehensive win lifted the Blues above Tottenham into fourth, with just one point separating the top five.
United stay seventh as the gap between them and the early pacesetters widens to six points, with almost a quarter of the season gone.
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'You're not special anymore!'
Mourinho was making his first return to Stamford Bridge since he was sacked in December 2015, leaving when the defending champions were 16th in the Premier League.
The Portuguese boss said before the match he was unsure - and also unconcerned - about the reception he would be given by the home fans.
Unsurprisingly, for a man who delivered seven trophies in his two spells at the club, it turned out to be largely positive.
Hundreds gathered to greet Mourinho as he walked off the United team bus, while he received a warm embrace from former skipper John Terry before kick-off.
That is where the Blues' hospitality ended. Instead, it was his United team who provided the generosity.
And the home fans could not resist a cheeky dig at their former boss - chanting "you're not special anymore" to the self-proclaimed Special One.
Jose Mourinho
Mourinho (top right) was the focus of attention as he arrived at Stamford Bridge
United exposed by defensive horror show
Mourinho said before the game he would not "celebrate like a crazy kid" if his new team scored at Stamford Bridge.
Unfortunately for him, there was little sign of that resolve being put to the test as Chelsea capitalised on his side's defensive deficiencies.
United were ragged at the back, ripped open by Pedro's opener without even having a touch and left flapping from then on.
And the severity of the scoreline was emphasised by some startling statistics:
It was the heaviest defeat for Jose Mourinho in all competitions since Real Madrid's 5-0 defeat by Barcelona in November 2010.
United suffered their heaviest Premier League defeat since the 6-1 loss against Manchester City in October 2011.
The Red Devils lost a Premier League away match by four or more goals for the first time since 1999 - also at Chelsea.
The tone was set when Chris Smalling hesitated in dealing with a long ball, allowing Pedro to nip in and round David de Gea - and United never recovered.
The basic ability to defend a set-piece eluded them for Chelsea's second, two deflected touches helping the ball on to the unmarked Cahill, who lashed in.
United, without the injured Wayne Rooney, looked unrecognisable from the side that dug in to keep a clean sheet at Anfield on Monday. They looked unrecognisable from any Mourinho side, always expected to be well organised and difficult to break down.
A stony-faced Mourinho watched his static defenders allow Hazard and then Kante to skip into space and score after the break, leaving new Chelsea manager Antonio Conte lapping up the acclaim of a home crowd revelling in their former boss's misfortune.
"We made incredible defensive mistakes," said Mourinho. "And then you pay for that."
Chelsea beat Manchester United
Chelsea's shots-at-goal graphic shows how clinical they were against United, scoring with four of their six on-target efforts...
Chelsea beat Manchester United
...while United's paints a different picture. They had five shots on goal but none of them forced Chelsea keeper Thibaut Courtois into an outstanding save.
Conte starting to shine
former Italy manager Antonio Conte was tasked with restoring Chelsea's fortunes after a chastening season, which started under Mourinho's leadership and ended in the defending champions finishing 10th.
Conte's start has not been smooth, however.
Back-to-back league defeats against Liverpool and Arsenal last month led to some criticism, forcing him to laugh off rumours of his imminent sacking.
But, after making a tactical switch to a 3-5-2 formation following those defeats, Conte has seen his team flourish.
Three successive victories have pushed the Blues back among the frontrunners, with the Stamford Bridge crowd showing their appreciation for the new manager as he urged them to make more noise during the second half.
"It is important for the manager to find the right solution for your team," said Conte on his team's recent revival.
"It was not a good situation when Chelsea conceded in every game - we change and now we are playing good football."
Chelsea beat Manchester United
Mourinho and Conte exchanged words as they shook hands at the final whistle
Man of the match - Eden Hazard (Chelsea)
Eden Hazard
Belgium star Hazard lost his way under Mourinho last season, but showed his former boss that he is returning to his best form with an assist, a goal, and a constant threat to the United defence
Post-match reaction
Chelsea manager Antonio Conte:
"We made a fantastic start. We scored the early goal but we continued to play good football, intensity and possession, create many chances. Today we didn't concede, which was important. It was a type of win that increases the confidence.
"We wanted to show our ambition and give the satisfaction to our fans and show last season was very bad."
Manchester United manager Jose Mourinho:
"You come with a strategy, you cannot concede a goal in the way we did.
"We were coming to have an offensive approach. We wanted to create chances; we showed that after the 1-0. The second and the third were counter-attack goals.
"It is one of those days when you give the advantage to opponents by doing nothing.
"In terms of points, we got zero points, we lose three points. We are six points from the top, three from the top four, we now need to win matches. We need to win our matches now, which are not easy.
"We need to win to close that gap - after these last three matches, we made two out of nine. We now need points."
What's next?
Next comes the not-so-small matter of midweek derby matches for both teams.
Chelsea travel across London to face West Ham in the EFL Cup fourth round on Wednesday (19:45 BST), while Manchester United host neighbours City at Old Trafford in the same competition on the same day (20:00 BST).
Line-ups
Match Stats
Live Text
Home TeamChelseaAway TeamMan Utd
Possession
Home44%
Away56%
Shots
Home14
Away16
Shots on Target
Home6
Away5
Corners
Home5
Away7
Fouls
Home7
Away17
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Topic: Jose Mourinho was humiliated on his return to Chelsea as his former club blew aw (Read 1156 times)
