Jose Mourinho's side were extremely poor in the first half of the game as Étienne Capoue gave the hosts a deserved lead at half time.
In the second half, United were a tiny bit better and got an equaliser from Marcus Rashford. This was as good as it got though for the visitors as late goals from Juan Camilo Zúñiga and Troy Deeney gave the hosts a huge victory.
Another slow start from the visitors as Watford miss chances
For the third consecutive game in a row, United started poorly as the hosts went extremely close on two occasions in the first 15 minutes of the game.
The first chance came from a free-kick which defender Sebastian Prödl headed dangerously back across goal that Deeney was inches away from tapping the ball into the net.
The second chance was the one though that really should have been scored. A long ball was launched into the box which David de Gea dropped after Chris Smalling clattered into him. The ball then fell to Odion Ighalo, who had an open goal at his mercy, but he somehow put his shot wide of the goal.
Deeney had Watford's next chance not long after as his header forced De Gea into making a great diving save to keep his side in the game.
United were slow in growing into the game but finally had their first sight of goal midway through the half and really should have taken the lead. Zlatan Ibrahimovic started a quick counter attack on the half way line which led to a cross from Rashford which found it's way too Ibrahimovic at the back post but somehow he put his left-footed shot wide of the goal when he should have hit the target at least.
The visitors continued to press and were so unlucky to not take the lead when Paul Pogba let a shot go from fully 30 yards out and was so unlucky to see his dipping shot come back off the bar.
Capoue scored his fourth Premier League goal of the season
The home side though grew back into the game after this and took a deserved lead 10 minutes before half-time. Capoue scored the goal from a great cut back from Daryl Janmaat, who won the ball from Anthony Martial in the wide area, to score his fourth Premier League goal of the season.
United pushed forward looking for an equaliser before half time but failed to do as once again their inability to start games well was the main problem in the first half.
Rashford equalised just after the hour mark
Just as it looked like Watford were starting to take control of the second half as United started poorly, the visitors managed to get a crucial equaliser just after the hour mark.
Ibrahimovic's cross into the box caused the home side problems and it dropped to Rashford, who managed to squeeze the ball the past Heurelho Gomes to get a really important goal for the side.
United continued after this to press for a winner and they almost got one when a floated cross from Marouane Fellaini found Ibrahimovic in the box and the big striker powered his header towards goal and it took a fantastic save from Gomes to keep it out.
Zuniga and Deeney score late on to give Watford a huge victory
The save from Gomes was crucial as the hosts went down the other end a few minutes later and got themselves back in the lead. Zúñiga had only been the pitch for one minute as he smashed the ball home past De Gea after a brilliant cut back from Roberto Pereyra found the substitute in the box.
It got even worse for United before the end when Fellaini fouled Zuniga in the box and referee Michael Oliver straight away pointed to the spot. Deeney stepped up to take and made no mistake by smashing the ball into the top corner of the net to give his side all three points and condemn United to three consecutive losses.