Chris Eagles, who was on loan with Sheffield Wednesday earlier in the season, opened the scoring.
Marlon King grabbed two to take his season's tally to 15 and substitute Hameur Bouazza added the fourth, with Paul Ifill netting for the home side.
Fourth placed Watford, boosted by four straight wins, forced the early pace before snatching the lead in the sixth minute when they dissected the United defence.
King cleverly picked out Darius Henderson in space on the left flank. He nipped past full-back Derek Geary before pumping in a low cross met by Eagles, who beat Paddy Kenny with a spectacular diving header from eight yards.
United were down to ten men in the 25th minute as David Unsworth suffered a rush of blood after Eagles slid in to challenge for a loose ball.
The ex-Portsmouth defender shoved out his hand to clumsily push the youngster in the face and received a straight red card.
Henderson saw his snap-shot scooped up by Kenny before Watford went 2-0 up 66 seconds after the re-start following a miserable blunder by Leigh Bromby four yards outside the United box.
The centre-back moved in to shield King as the pair chased Matthew Spring's throughball, but the nervous Bromby dithered and the Watford hitman nipped in to snatch the ball and beat the United keeper with a low drive.
United were back in the game in the 58th minute when veteran defender Craig Short stretched beyond Malky Mackay to meet Chris Armstrong's cross and Ifill stuck out a foot to toe-poke the ball past visiting goalkeeper Ben Foster.
Kenny produced another stunning save to deny Ashley Young before King snatched his second of the night in the 69th minute with a right foot strike from 15 yards.
Bouazza then took advantage of another Bromby blunder to add the fourth two minutes from time.