A first-half Jay DeMerit header and a second-half Neil Sullivan own goal put the Hornets in control before a late Darius Henderson spot kick sealed Watford's passage into the Premiership.
After a frantic start, the game settled down and there didn't appear to be too much quality on show as Leeds had to deal with Watford's route one style. Both sides appeared to be cancelling each other out until Watford broke the deadlock in the 25th minute through a DeMerit header. The American centre-back powered in a close-range header from a corner to give Watford a lead they just about deserved.
From that moment on Watford tightened their grip on the game as Leeds found it hard to create any meaningful chances and their best opportunity of the half came in stoppage time when Eddie Lewis curled a free-kick just over Ben Foster's crossbar.
The second half started in a similar fashion to the first with neither team really troubling the other until Watford won a throw in deep in the Leeds half. The long throw was flicked on in the box and fell to James Chambers who hit a shot on the turn that didn't look to be troubling Sullivan. But as the ball bounced in front of the Leeds keeper, it spun wickedly and bounced off the post and in off the stranded stopper.
Leeds were dead and buried on 84 minutes when Darius Henderson rolled in a spot kick after Derry had tripped Marlon King as he shaped to shoot.
So Watford will be playing top flight football once again next season and 35-year-old Aidy Boothroyd will become the Premiership's youngest manager. Leeds will remain in the Championship for a third successive year. The full 24 teams competing in the division next season will be known on Saturday after the League One play-off final contested between Barnsley and Swansea.