Arsenal's 12-match home winning streak in the Premier League came to a shuddering halt as Jesse Lingard scored twice in an exhilarating 3-1 victory for second-placed Manchester United on Saturday.
Skipper Antonio Valencia and Lingard punished woeful Arsenal defending to put United two ahead inside 11 minutes at the Emirates and Lingard struck again in the 63rd minute after Alexandre Lacazette's goal had revived home hopes.
Keeper David De Gea was inspired for United who had to play the final 15 minutes with 10 men after record signing Paul Pogba was shown a straight red card for a reckless 'studs up' foul on Arsenal right back Hector Bellerin.
Midfield powerhouse Pogba will now miss next weekend's derby against leaders Manchester City.
United's victory cut City's lead to five points, although they host struggling West Ham United on Sunday.
Mourinho has been criticised for his negative tactics on the road against the big guns and United had managed only one goal in seven against 'top-six' clubs since he arrived.
There could be no criticism this time as his side, unchanged from the one that beat Watford 4-2 on Tuesday, harried and hounded Arsenal in a devastating opening spell.
They took the lead in the fourth minute when Laurent Koscielny's dreadful pass saw United seize possession and Valencia played a one-two with Pogba before drilling a low shot through the legs of Petr Cech.
Arsenal failed to learn their lesson and seven minutes later Shkodran Mustafi was dispossessed by Lingard before Romelu Lukaku and Anthony Martial combined to set up Lingard to beat Cech.
Mustafi limped off almost immediately.
Arsenal finally roused themselves and United's goal led something of a charmed life for the rest of a lively first half.
Lacazette was guilty of over-elaboration as his shot was deflected against the bar by United keeper David De Gea with Granit Xhaka driving the loose ball wide.
De Gea made smart saves from Hector Bellerin and Sead Kolasinac shortly before halftime and reacted superbly to spare Lukaku's blushes after the striker, back defending a corner, made a hash of an attempted clearance.
Arsenal made the perfect start to the second half with Lacazette firing home from Aaron Ramsey's knockdown.
A wildly open encounter then saw Lingard through alone but his shot was half-stopped by Cech before looping up in slow-motion and striking the post with Martial's follow-up blocked.
De Gea then surpassed himself, diving to stop Lacazette's skidding low shot and then stretching out a leg to keep out Alexis Sanchez's follow-up effort.
United's pace and power was always a threat and they counter-attacked in deadly fashion in the 63rd minute with Lukaku again involved, holding the ball up before finding Pogba. The Frenchman brushed off Koscielny's feeble challenge and rolled the ball across the area for Lingard to tap home.
Pogba blotted his copybook soon after though and will be a big miss for next weekend's Old Trafford blockbuster.