Preston started brightly with Wallace getting his first league start and it was hiss cross which led to Preston's goal. Mawene managed to head back across goal for St Ledger to place in the net for the lead after five minutes.
The lead did not last as ex Burnley full back Jon Harley swept in a sweet left foot shot to level the scores after eleven minutes. Watford then scored what turned out to be the winner after 22 minutes when Tommy Smith ghosted into the box forcing Lonergan into a save but the Watford man poked home the rebound.
Watford controlled the rest of the first half with Lonergan forced to make several saves to keep the visitors in the game.
Preston came out stronger in the second half with Elliott replacing Mellor and the former Wolves man was gifted a glorious chance but shot straight at the keeper. Preston despite having a lot of the ball failed to convert which has been the story of the season.
Irvine now has two weeks to get some confidence back into his players before they end up in another relegation battle.