19-year-old Joao Pedro tucked away a second-half penalty to secure all three points for the travelling Hornets.
Watford dominated large parts of the game as Preston were unable to test goalkeeper, Daniel Bachmann, registering no shots on target.
Story of the match
From the start of the game, Watford dominated the ball, having large spells of possession in both halves of the pitch.
It took them just two minutes to get in behind the Preston defence but a lack of quality in the pullback from Ken Sema saw the ball intercepted at the last minute.
As Watford continued to have the larger share of possession, Preston began to sit deep in numbers and looked to hit Watford on the break.
January signing Ched Evans had two glorious opportunities for the home side in the 11th and 17th minute.
He blazed his first effort well over the bar after a lovely ball over the top from Ben Whiteman put him in behind the Watford backline.
The second attempt just whistled past the near post after he twisted and turned past Francisco Sierralta but he was unable to test goalkeeper, Daniel Bachmann.
Watford then began to settle into the game had a wave of attacks in the later stages of the half. The dangerous Ismaila Sarr shot tamely over the bar in the 23rd minute with wide players Sema and Kiko Femenia looking a constant threat down either flank.
The Hornet's final pass, touch and shot in the final third lacked any real quality and guile despite them picking up some really positive attacking positions.
Even though Watford dominated the majority of the first period, they were unable to register a shot on target and the home side had the better opportunities.
Preston goalkeeper Daniel Iversen was very well protected by a strong, resolute and organised Lilywhite back four who cleared and blocked everything.
The second half started in the same fashion as the first. Watford came out of the blocks fast and looked to attack in numbers.
Their perseverance paid off in the 51st minute when the Hornets were delicately passing the ball around the Preston penalty area and after a clumsy challenge from Tom Barkhuizen on Joao Pedro, Xisco Munoz's men were awarded their ninth penalty of the season.
It was Pedro who stepped up and he made no mistake in firing the penalty into the bottom left-hand corner and past Iversen who dived the right way.
After the Watford goal, it was Preston's turn to have more of the ball. They had a succession of attacks and corners but they failed to trouble Bachmann in the Hornets net.
Preston had much more intent with their possession and forced Munoz's men into some alert defending.
Space looked to open up for Femenia in the 62nd minute as Watford looked to hit Preston on the counter but he fired his shot well wide.
After the 65th minute, the game went a little bit flat with neither team dominating or creating any meaningful opportunities.
The next most notable chance came in the 75th minute when Tom Cleverly struck a free-kick from 30 yards out which just sailed over the bar.
In the 82nd minute, Emil Riis Jakobsen shot just wide for Preston after a scramble in the box as they began piling on the pressure in the closing stages.
Despite their best efforts, Preston were unable to rescue a point as Watford defended everything that the Lilywhites threw at them.
Stand out player - Craig Cathcart
Craig Cathcart was a wall at the back for the Hornets this evening as he helped them to their 15th clean sheet of the season.
Cathcart had a pass success rate of 86%, he won four duals, made four clearances and eight recoveries.
In the latter stages of the game when Preston began to throw the kitchen sink at Watford, he was alert, strong and resolute in making sure they did not breakthrough.
Takeaways from the match
Both teams can defend
Both teams defended well in large parts this evening. Preston were excellently organised in the first half and really restricted Watford to long-range efforts which were no trouble for Iversen.
Despite conceding a penalty in the second half, they were mainly very hard to break down for Watford.
The Hornets then proved why they are so high up in the table in the second half, poaching an early penalty and then seeing out the game fairly comfortably for the rest of the half.
Ken Sema is lively
Wide man, Sema was lively all evening for Watford. He was a real handful for the Preston defence and was constantly on the front foot.
Sema had 42 touches and he stretched the play when Watford were defending, offering them an out ball. His pace and directness certainly scared the Lilywhites defence.