The Hornets were behind after only six minutes when captain Mowatt found the net with a spectacular long-range drive.
Tom Cleverley and Ben Wilmot both missed glorious chances as Vladimir Ivic’s Watford failed to even register a shot on target.
Story of the match
Barnsley were hoping to back up the fantastic start to Ismael’s reign in midweek, when they dismantled Queens Park Rangers, and they made the perfect start with a goal after just six minutes.
It came from their high press forcing a Watford error, which allowed Mowatt to advance with the ball. Instead of looking to pick a pass, he let fly from over 25 yards out, and Ben Foster had no hope of getting close to the rasping drive that bounced off the post and into the back of his net.
Watford were struggling to settle into the game, their best outlet being Ismaila Sarr who peeled away dangerously on a few occasions up front. He almost set up a goal for strike partner Joao Pedro with a ball across to the far post, but goalkeeper Jack Walton blocked the effort.
But it was Barnsley making the best moves as they looked to get forward at speed, with Mowatt volleying Conor Chaplin’s cross over the bar and Chaplin then forcing a Foster save from a slick one-touch move and Callum Brittain’s cross.
Having failed to settle into a rhythm in the first half, Watford came out after the break looking much sharper. Their first big chance was a golden one for Cleverley, who blazed the ball over when found free in the middle of the box following a brilliant individual run from Sarr.
Chances were starting to come more freely for both teams, with Wilmot heading over from a corner and George Miller striking wide when played through on goal moments after coming off the bench.
And if Cleverley’s miss was an improbable one, hands were firmly on heads when Wilmot was found again from a set-piece yet managed to head into the ground and over the crossbar from close range.
Both Andre Gray and Will Hughes made long-awaited comebacks off the bench for the Hornets, who changed formation to a front three to try and rescue the game, but Barnsley’s impressive rearguard continued to defy them as they hung on for an impressive win.
Ismael’s Barnsley the real deal
The impact that Ismael has had on this Barnsley team in a short space of time is staggering, with the physical and technical demands of his style being implemented almost to the letter. The man himself said his side had been “perfect”.
The shape and high press are familiar from the reign of predecessor Gerhard Struber but many new ideas have been brought in by the Frenchman, not least an even more energetic game and a very direct one.
In possession, the focus is on speed and direction. There’s no knocking it about in defence – the ball goes forward, and usually from back to front in one or two sharp passes.
Add that to the polish on their established pressing game from the back three to the front three, and it’s already making for an eclectic and exciting mix.
Watford frontline needs refresh
There were several areas of concern for Watford in this game, including the difficulties early on playing out from the back, and the struggle of the high-calibre but perhaps too conservative midfield of Etienne Capoue, Nathaniel Chalobah and Cleverley.
The one that has been bubbling away for a while but was particularly notable tonight was the shortcomings of the chosen forward pair of Sarr and Pedro.
The 19-year-old Pedro has shown real potential so far this season but couldn’t get involved in this game, while Sarr was busy when coming into his more natural position out wide. Both have quality but as a strikeforce it isn’t working.
It has largely been a marriage of convenience in the absence of Gray and Troy Deeney. The former returned here and the latter should do likewise soon, though it remains to be seen whether the co-occurrence of their comebacks and Watford’s loss of form will force Ivic into a change of system as well as personnel.
Man of the match: Alex Mowatt (Barnsley)
Any of Barnsley’s central defenders could take this prize, as well as the ever-versatile Callum Styles, but Barnsley’s skipper was once again immense in the middle of the pitch. His goal wasn’t bad, either.