Going into the game on the back of a derby defeat at home to Manchester City and a Europa League loss at Feyenoord, United were largely disappointing as Mourinho slipped to three successive defeats as a manager for the first time in over a decade.
The in-form Etienne Capoue struck his fourth goal in six games to put the hosts ahead at Vicarage Road and although Marcus Rashford levelled on 62 minutes with United's first shot on target, Camilo Zuniga came off the bench and scored almost immediately before a late Troy Deeney penalty secured a memorable win for the Hornets - their first victory over United since 1986.
TWEET OF THE MATCH
"#WATMUN We can see you sneaking out! Watford fans good banter," ex-Manchester City striker Shaun Goater ( @OfficialSGoater) enjoyed the taunts directed at the United fans
STAR MAN
Troy Deeney
The Watford captain was superb as he led the line, bullying the United defence, as well as doing the dirty work in defence.
While his United counterpart Wayne Rooney endured a miserable afternoon, Deeney showed that with plenty of fight he can enjoy as good a season as last year and the injury-time penalty was just reward for an afternoon of toil.
MOMENT OF THE MATCH
What a moment to savour for Zuniga as the man on loan from Napoli came off the bench to have a near-instant impact, slamming Watford back in front with a controlled finish.
The Colombia international had not scored in five years but this strike will live long in the memory for everyone involved with Watford as they recorded a rare victory over United.
VIEW FROM THE BENCH
Mourinho looked to change United's shape as he opted for a 4-3-3 formation, with Rooney and Paul Pogba joining Marouane Fellaini in the middle of the park.
That meant Anthony Martial, Rashford and Zlatan Ibrahimovic would form a potentially potent strikeforce, but it was not to be.
Both Rooney and Pogba under-performed, despite the latter hitting the crossbar with a first-half rocket, while Ibrahimovic was kept quiet by Sebastian Prodl and Martial was forced off with injury.
RATINGS
Watford
Heurelho Gomes 6
Craig Cathcart 6
Sebastian Prodl 8
Miguel Britos 7
Jose Holebas 7
Daryl Janmaat 6
Etienne Capoue 7
Valon Behrami 7
Roberto Pereyra 6
Troy Deeeney 8
Odion Ighalo 7
Substitutes
Nordin Amrabat (for Janmaat) 6
Camilo Zuniga (for Capoue) 8
Isaac Success (for Ighalo) 5
Manchester United
David de Gea 7
Antonio Valencia 6
Eric Bailly 5
Chris Smalling 6
Luke Shaw 6
Marouane Fellaini 6
Paul Pogba 5
Wayne Rooney 4
Anthony Martial 5
Zlatan Ibrahimovic 5
Marcus Rashford 7
Substitutes
Ashley Young (for Martial) 6
Juan Mata (for Valencia) 5
Memphis Depay (for Shaw) 5
WHO'S UP NEXT?
Burnley v Watford (Premier League, September 26)
Northampton v Man Utd (EFL Cup, September 21)
Source : PA
Source: PA