Second half goals from Ismaila Sarr and Troy Deeney proved the difference between the two sides as the Hornets gave their survival chances a huge boost, and put a dent in United’s top four bid.
Watford put in an improved performance last time out in defeat to Liverpool in Pearson’s first game in charge and there were encouraging signs again in the opening exchanges, although it took a while for the game to really get going.
The first real opportunity came after the half hour mark as Anthony Martial threaded the ball through to Jesse Lingard but he fired his shot just over the crossbar from the edge of the area.
Watford made the breakthrough on 50 minutes thanks largely to a horrendous error from visiting goalkeeper David De Gea.
Will Hughes’ free kick into the area found its way through to Sarr. His scuffed volley should have resulted in a routine save for the Spaniard but the ball somehow went through his hands and landed in the back of the goal, via a hit off De Gea’s face.
It went from bad to worse soon after for United as Sarr was upended inside the penalty area by Aaron Wan-Bissaka. Troy Deeney smashed the resulting penalty straight down the middle to give Watford a two-goal cushion on 54 minutes.
United tried to get back into the game and Wan-Bissaka drew a good save from Ben Foster in the Hornets goal as he attempted to atone for his error.
Foster was also on hand to deny Harry Maguire, Paul Pogba and Marcus Rashford late on as Watford stood firm to pick up a vital three points.
Watford remain bottom of the Premier League table due to an inferior goal difference to Norwich immediately above them, with Pearson’s side now six points adrift of safety.
United are in eighth place and they could be up to seven points adrift of the top four after Chelsea’s trip to Spurs has finished on Sunday.