England worldwide Young netted twice in six first-half minutes at a freezing Vicarage Road on Tuesday, with Anthony Martial and Jesse Lingard also on target.
Watford have lost 12 of their last 13 games against Manchester United in all competitions, winning the other in the Premier League in this exact fixture last season (3-1).
Young had seen his deflected match-winner against Brighton and Hove Albion chalked up as an own goal by Lewis Dunk, but there was no denying him his brace against the club where he made his name.
Watford rallied, however, with Troy Deeney pulling one back from the penalty spot before Abdoulaye Doucoure turned home with six minutes remaining.
Manchester United midfielder Nemanja Matic was forced off early in the second half with what appeared to be a leg injury. 32-year-old Ashley Young has been the club's first choice left back in recent weeks, with Luke Shaw, Matteo Darmian and Daley Blind seemingly not fancied by the Portuguese, and likely to be allowed to leave the club in the near future.
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A foul on Paul Pogba by Doucoure gave United a free-kick 25 yards from goal in a central position and Young whipped a lovely effort into the top-left corner.
United continued their onslaught as Martial latched on to a Lukaku through-ball following a poor attempted clearance from Christian Kabasele to finish low past Gomes.
Drawing one back, the home side went looking for more and managed a second after 84 minutes.
The Hornets ended the half on a high as Femenia burst forward and clipped a cross towards Richarlison, who headed against David De Gea's crossbar. If I go back then I'll be a better player.
It should have been even worse for Watford at the break, but Gomes saved well from Lukaku before jumping to his feet to make an even better stop from Lingard's follow-up effort.