Chelsea were flying out of the traps in the first half, with Pedro scoring after just 30 seconds with Gary Cahill adding a second to round off a great first-half.
It got worse for the Red Devils, as Eden Hazard added a third after the hour mark, before N'Golo Kante rounded off a excellent home performance from the Blues.
An excellent start
Many inside will have welcomed Mourinho as he stepped out back on his old stomping ground, but it wasn't the same on the pitch as Chelsea quickly took the lead.
It took less than a minute for them to find the net, as Marco Alonso fired a simple ball over the top looking for Pedro. David De Gea came flying off his line, followed by two defenders, but the Spaniard snuck in rounded his compatriot De Gea and stuck it into the empty net to make it 1-0.
United were shook by the early effort, but looked to get themselves back into the match. Zlatan Ibrahimovic came close as he got to Antonio Valencia's cross but he could only head over.
Hazard had his first chance in the 15th minute, as he took his chance first time on the edge of the box. The shot had De Gea rooted but it was just wide of the mark.
Doubling it up
Despite a slight improvement, United were knocked back once again as Chelsea added their second.
It was too easy for Antonio Conte's side, as a corner was flicked on into the path of Cahill, and it was simple enough for the defender to strike home from six yards.
Thibaut Courtois was almost a villain as he parried a shot into the path of Jesse Lingard but he did well to block the follow up effort as Chelsea maintained their control of the game.
They almost added a third in stoppage time. The ball was chipped into the area for Diego Costa who got ahead of Eric Bailly, but it was blocked well.
United looked up for the second period, with Lingard having a good effort, but the match was over when Chelsea added their third goal.
Again it was poor from United, as Nemanja Matic fizzed a ball into Hazard, the Belgian did well to control and check inside and fire it low into the net with De Gea unable to prevent it finding the bottom corner despite getting a palm to the ball.
Making a mockery
It was all but over, but Chelsea still had another goal in them with a lot of help from the United defence.
Kante received no pressure from Ander Herrera and Paul Pogba as he ran through, and his fake shot had Chris Smalling bamboozled before the Frenchman slotted home for his first goal for Chelsea.
Looking for a consolation
It was a terrible performance all round from Mourinho's men, but they had good chances to grab some sort of consolation.
Their best chance fell to Ibrahimovic, as he took it on chest and swivelled, but Courtois did well to get down to save.
The final chance came in second half stoppage time, it screamed desperation as Marcos Rojo tried one from distance but it was easy for Courtois to collect as United's miserable afternoon eventually came to an end.