Hornets manager Gianfranco Zola has assembled a side largely made up of players borrowed from Udinese but they clearly have more than enough ability to make an impression in the Championship.
The caveat to this win was that the goals they scored from Matej Vydra, Almen Abdi, Mark Yeates, Sean Murray and Troy Deeney all came after Leeds had been reduced to nine men, but prior to that they had still looked the better side, having gone ahead through Vydra.
Leeds did score a penalty through Michael Tonge but having lost Jason Pearce to a first-half red card and Rodolph Austin to injury after all their substitutions had been made, were made to suffer for the best part of an hour.
Watford opened the scoring on 27 minutes, with Alex Geijo collecting the ball on the break just inside the Leeds half and promptly sending a pass ahead for Vydra to chase. There was only one winner in his footrace with Lee Peltier but his chance still needed finishing, something he did with a clinical edge-of-the-box drive.
The home side were simply unable to handle the speed with which Watford were attacking them and on the stroke of half-time, they again paid the price. El-Hadji Diouf's pass sold Pearce shot and as Abdi nipped in to rob possession, the former Portsmouth defender poleaxed him and was duly dismissed by referee Trevor Kettle.
Warnock's response was to make three changes by the time the second half had started, but his cavalier approach came back to haunt him two minutes after it when Austin went down in a heap and had to be carried off, leaving United to carry on with nine men.
Against slim numbers it was hard to gauge how good Watford were playing but there was no doubting the quality of their third - a brilliant 25-yard free-kick from Yeates which again gave Kenny no chance.
Somehow, Leeds got on the scoresheet themselves in the 79th minute when Varney was upended by Joel Ekstrand, allowing Tonge to score his fourth of the season, but it was a mere blip for Watford.
With eight minutes left Vydra coolly chipped a counter-attack second after an excellent ball from Anya and then, deep into the 10 minutes of added time, Murray placed a 25-yard drive beyond Kenny.
That appeared to have rounded things off but there was still more time for Watford to score again, Deeney replicating Murray's effort.