Goals from Billy Jones and Ross Wallace stung the Hormets after a slow start for Alan Irvine's side.
North End were second-best for much of the first half and were lucky not to fall behind when John-Joe O'Toole headed against the crossbar and Darren Ward's follow-up effort was blocked on the line by Jay McEveley's thigh.
But once makeshift midfielder Jones had headed Preston in front a minute before half-time Irvine's men grew in confidence and looked a different side in the second half.
Wallace doubled the lead after 68 minutes, cutting in from the right on to his left foot and lashing in a shot which keeper Richard Lee could only help into the net.
The win lifted North End back into the top half of the table, just two points off a play-off place and with lowly Southampton next up at Deepdale on Saturday.
A power cut on Lowthorpe Road left the new Invincibles Pavilion in darkness and blacked out the big screen as the game got underway.
Watford carved out the first chance of the night after six minutes when a right wing corner was met by Darren Ward, but the defender's downward header was saved by Andrew Lonergan diving to his left.
Ross Wallace created Preston's first opening three minutes later racing from his own half to the left edge of the visitors' box before crossing for Jay McEveley to miscue his header as he thundered in.
Wallace's inswinging corner from the right caused problems for the Watford defence with Youl Mawene's stooping header that blocked at the near post and then hoofed away to safety.
North End had a lucky escape after 20 minutes when Lonergan appeared to be pushed as he came to punch a corner from the right, John-Joe O'Toole put his close-range header against the crossbar and McEveley blocked Ward's follow-up effort on the line with his thigh before booting clear.
North End went close six minutes before the break when they broke from a goalmouth scramble in their own box for Stephen Elliott to fire in a shot which was only inches wide of the left-hand post with keeper Richard Lee beaten as he scrambled across.
The breakthrough was not long in coming as Preston finally begam to find their feet after a spell playing second fiddle.
Wallace angled a great diagonal ball to Nicholson on the left of the box, he nodded back inside, the keeper got a hand to turn the ball up into the air and Jones surged forward to head in from close range.
The goal a minute before the interval gave Preston a much-needed confidence boost and they emerged for the second half looking revitalised.
Jones was close to doubling his and the team's tally after 54 minutes but St Ledger's cross caught under his feet in front of goal.
Paul McKenna was only just wide from distance and Youl Mawene sent a glancing header beyond the far post.
McEveley joined the attack with a surging run from his own half only to lift his shot over the top when he reached the edge of the Watford box.
And with wide players Wallace and Nicholson having switched wings, the on loan Sunderland winger suddenly started to look a real threat cutting in on to his left foot from the right side.
His first attempt after 67 minutes, hammered in low, thumped against the foot of the near post with the keeper beaten.
But less than 60 seconds later Wallace got it inch-perfect, turning inside his full-back once more before lashing in a left-foot shot, Lee only able to get a hand to it and turn it high into the net for Preston's second.
New loan signing Brown was thrown straight in at the deep end as manager Alan Irvine shuffled his pack to allow the experienced Hull City centre-half to go straight into the heart of the back four.
Sean St Ledger moved to right-back and Jones was switched to the centre of midfield, with Barry Nicholson moving out to the right-hand side and Chris Sedgwick dropping to the bench.
As expected Watford boss Aidy Boothroyd swung the axe in defence after his side leaked three in Saturday's home defeat by Wolves with Leigh Bromby and Adrian Marriapa stepping down to be replaced by Jay DeMerit and Ward.
PNE: Lonergan, St Ledger, Mawene, W Brown, McEveley, Nicholson, Jones, McKenna, Wallace, Parkin, Elliott.
Substitutes: C Neal, Jarrett, Sedgwick, Whaley, Neal.
Watford: Lee, Sadler, DeMerit, Ward, Doyley, Eustace, Harley, Smith, O'Toole, Rasiak,
Hoskins.
Substitutes: Tyler, McAnuff, Williamson, Ainsworth, Marriapa.
Referee: Mike Oliver (Northumberland).
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