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.
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.