Relegated Preston came from behind in their Championship farewell to finish one place off the bottom of the table.
Mid-table Watford went ahead after Danny Graham crossed for on-loan striker Andreas Weimann, and the 19-year-old found the bottom corner.
Barry Nicholson equalised for Preston with a long-range strike.
After the break Iain Hume's free-kick found the bottom corner and substitute Jamie Proctor sealed Preston's 10th league win of the season.
North End had gone into the match rooted to the foot of the table after only one victory in seven games.
They fell behind when on-loan Aston Villa striker Weimann fired a low shot past goalkeeper Iain Turner.
Watford defender Martin Taylor, who seemed to have recovered from a foot injury to maintain his record of starting in every league game this season, could only last 22 minutes and was replaced by Tommy Hoban.
Nicholson levelled with a 30-yard effort that eluded Hornets goalkeeper Scott Loach, with Keith Treacy's left-footed strike hitting a post soon after the restart.
Weimann was thwarted by Turner at the start of the second half before Treacy was denied by the woodwork again in the 54th minute, this time hitting the crossbar.
Hume put the home side ahead with his 12th goal of the season, driving in a 25-yard set piece.
Watford, the league's highest scorers away from home, twice went close to equalising, substitute Matthew Whichelow heading over from close range and Graham foiled from six yards by Turner.
North End's 11-season stint in the second tier ended on a positive note when Proctor claimed his first professional goal from close range after being set up by Treacy.