Hassell, on as a sixth-minute replacement for the injured Rory Delap, scored his first league goal since scoring for the Tykes in a 1-1 home draw with Swansea on 22 January, 2011.
But they lacked their normal attacking flair and rhythm and created little.
The Hornets - the Championship's highest scorers - had the better of the first half hour and home goalkeeper Luke Steele had to tip away Jonathan Hogg's header while Matej Vydra blasted a 20-yard shot over the bar.
Barnsley had their first real effort on goal on 33 minutes when Jason Scotland forced a good save from Jonathan Bond.
And two minutes later, the Tykes went ahead when Mellis found space on the left flank and his cross was headed into the bottom left-hand corner by Hassell.
Chris O'Grady almost made it 2-0 at the start of the second half but Bond saved at his near post.
Watford pressed late on but struggled to still struggled to create clear chances.
Vydra's volley forced a good stop from Steele, who also confidently dealt with a couple of late corners to ensure the home side's second win a week following the
victory over Brighton on Tuesday.
Barnsley manager David Flitcroft:
"I'm completely buzzing, we set out a game plan and it really worked.
"There's been a lot said about Watford and what they've tried to do and how they've done things. I think it's brilliant what they've done; I love creativity, I love the way that they've taken advantage of a situation.
"You've got to accept you're playing Watford, you're playing some top, top players and you've got to deal with them; we dealt with them.
"At the start of the season we got overrun by them and today we've got up close and personal. We've got in and around what they do, we've put them under real pressure and we've returned with three points and victorious, and it's an honour to manage this football team at this point.
"I don't know many times Watford have not scored (this season), and I'm going to look that up when I get back to the office. We've kept a clean sheet against Watford, this can't be underestimated; the size of this performance, the level, the hunger, the desire that I spoke about before the game, it was just evident."
Watford manager Gianfranco Zola:
"I'm disappointed for the result, I'm disappointed for the performance and that's it.
"But it ends there because sometimes you have to admit, when the opposition fights and plays the way they played, sometimes the better quality doesn't prevail - especially in the Championship when you have teams fighting for their lives, like Barnsley, and they make it difficult for you to play and you cannot overcome that.
"I think my players will learn a lot from today. For many of them, they didn't even know a game like this.
"It is the first time we have faced a team that has played and pressed us so much throughout the game. And as I said, many of them with learn. In my team I have a lot of young players and games like this will teach them a lot."
Full Time The final whistle goes and the game is over.
Corner taken right-footed by Geoffrey Mujangi Bia from the left by-line, save made by Luke Steele.
Corner taken right-footed by Geoffrey Mujangi Bia from the left by-line, Tomasz Cywka manages to make a clearance.
The offside flag is raised against Chris O'Grady. Free kick taken by Jonathan Bond.
Foul by Marco Cassetti on Tomasz Cywka, free kick awarded. Jim O'Brien takes the direct free kick.
Geoffrey Mujangi Bia takes a long range shot clearing the crossbar.
Chris O'Grady is flagged offside by the assistant referee. Jonathan Bond takes the free kick.
Chris O'Grady takes the chance to get an effort at goal. Jonathan Bond makes a comfortable save.
Jim O'Brien takes a short corner.
Chris Dagnall produces a right-footed shot from inside the area that clears the bar.
Nathaniel Chalobah gives away a free kick for an unfair challenge on Tomasz Cywka. Bobby Hassell takes the free kick.
Jonathan Bond takes the direct free kick.
Booking Booking for Scott Wiseman.
Scott Wiseman concedes a free kick for a foul on Lloyd Doyley.
Unfair challenge on Kelvin Etuhu by Geoffrey Mujangi Bia results in a free kick. Bobby Hassell restarts play with the free kick.
Chris Dagnall concedes a free kick for a foul on Joel Ekstrand. Nyron Nosworthy takes the direct free kick.
Shot by Matej Vydra. Luke Steele makes a brilliant save.
Corner taken by Cristian Battocchio from the right by-line to the near post, Bobby Hassell makes a clearance.
Free kick awarded for an unfair challenge on Tom Kennedy by Nathaniel Chalobah. Free kick taken by Luke Steele.
Nyron Nosworthy takes the direct free kick.
Booking The referee shows David Perkins a yellow card.
Unfair challenge on Fernando Forestieri by David Perkins results in a free kick.
Free kick awarded for a foul by Geoffrey Mujangi Bia on Scott Wiseman. Luke Steele takes the free kick.
Unfair challenge on Joel Ekstrand by Chris O'Grady results in a free kick. Free kick crossed right-footed by Nathaniel Chalobah from right channel.
Chris O'Grady is caught offside. Indirect free kick taken by Marco Cassetti.
Tom Kennedy restarts play with the free kick.
Substitution Fernando Forestieri joins the action as a substitute, replacing Ikechi Anya.
Substitution (Watford) makes a substitution, with Geoffrey Mujangi Bia coming on for Jonathan Hogg.
Foul by Nathaniel Chalobah on Kelvin Etuhu, free kick awarded.
Free kick taken by Luke Steele.
Booking The referee shows Troy Deeney a yellow card for unsporting behaviour.
Luke Steele fouled by Troy Deeney, the ref awards a free kick.
Direct free kick taken by Jonathan Bond.
Substitution Jacob Mellis leaves the field to be replaced by Tomasz Cywka.
Free kick awarded for a foul by Jacob Mellis on Joel Ekstrand.
Chris O'Grady takes a shot. Save by Jonathan Bond. Short corner worked by Jacob Mellis.
Inswinging corner taken right-footed by Tom Kennedy from the right by-line, Troy Deeney makes a clearance.
Unfair challenge on Luke Steele by Troy Deeney results in a free kick. Direct free kick taken by Luke Steele.
The game restarts for the second half.
Substitution Chris Dagnall joins the action as a substitute, replacing Jason Scotland.
Substitution (Watford) makes a substitution, with Nathaniel Chalobah coming on for Mark Yeates.
45:00 +2:40 Half time
Half Time The half-time whistle blows.
The ball is sent over by Jonathan Hogg, Header from deep inside the area by Matej Vydra goes over the bar.
Free kick awarded for an unfair challenge on Chris O'Grady by Joel Ekstrand. Tom Kennedy restarts play with the free kick.
Foul by Chris O'Grady on Nyron Nosworthy, free kick awarded. Jonathan Bond takes the direct free kick.
Free kick awarded for a foul by Lloyd Doyley on Chris O'Grady. The free kick is swung in right-footed by Jacob Mellis, clearance by Nyron Nosworthy.
Ikechi Anya crosses the ball, Tom Kennedy manages to make a clearance.
Assist on the goal came from Jacob Mellis.
34:33 Goal scored
Goal! - Bobby Hassell - Barnsley 1 - 0 Watford Bobby Hassell finds the back of the net with a headed goal from inside the area. Barnsley 1-0 Watford.
Free kick awarded for a foul by Marco Cassetti on Jacob Mellis. Jacob Mellis crosses the ball,
Jason Scotland has a drilled shot. Jonathan Bond makes a save.
Jason Scotland is penalised for handball and concedes a free kick. Jonathan Bond takes the direct free kick.
Free kick awarded for an unfair challenge on Jonathan Hogg by Jim O'Brien. Direct free kick taken by Jonathan Bond.
Matej Vydra produces a right-footed shot from just outside the area that goes harmlessly over the bar.
Effort by Bobby Hassell from a long way out goes over the crossbar.
Free kick awarded for a foul by Mark Yeates on Jason Scotland. David Perkins restarts play with the free kick.
Inswinging corner taken right-footed by Jacob Mellis from the left by-line, Lloyd Doyley fouled by Jason Scotland, the ref awards a free kick. Direct free kick taken by Jonathan Bond.
Booking Bobby Hassell is cautioned by the ref for unsporting behaviour.
Free kick awarded for a foul by Jacob Mellis on Jonathan Hogg. Mark Yeates crosses the ball from the free kick right-footed from right wing, clearance made by Bobby Hassell.
David Perkins takes the direct free kick.
Booking Joel Ekstrand is given a yellow card.
Foul by Joel Ekstrand on Jim O'Brien, free kick awarded.
Free kick awarded for an unfair challenge on Matej Vydra by Kelvin Etuhu. Jonathan Bond takes the direct free kick.
Mark Yeates gives away a free kick for an unfair challenge on Jim O'Brien. Free kick taken by David Perkins.
Foul by Chris O'Grady on Lloyd Doyley, free kick awarded. Free kick taken by Jonathan Bond.
Corner taken right-footed by Mark Yeates from the right by-line, save made by Luke Steele.
Kelvin Etuhu challenges Jonathan Hogg unfairly and gives away a free kick. Joel Ekstrand takes the free kick.
Substitution (Barnsley) makes a substitution, with Bobby Hassell coming on for Rory Delap.
Jonathan Hogg takes a shot. Luke Steele makes a save.
Free kick awarded for an unfair challenge on Chris O'Grady by Mark Yeates. Tom Kennedy crosses the ball in from the free kick, free kick awarded for an unfair challenge on Joel Ekstrand by Chris O'Grady. Jonathan Bond restarts play with the free kick.
Free kick awarded for an unfair challenge on Chris O'Grady by Joel Ekstrand. Tom Kennedy fires a strike on goal direct from the free kick.
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