The Greek wing-back struck a superb second-half strike from just outside the area which secured the Hornets all three points in a match lacking in any other moments of quality.
It was an intense start to the game, Deeney failing to get a shot off after fighting through the Boro defence, and Downing firing over for the hosts.
Gaston Ramirez also flashed a strike wide, as Boro started to show their dominance – a Behrami header was a rare chance for the Hornets.
The match died down heading to the 20 minute mark, with a lack of quality at either end of the pitch preventing any real chances; a dismissed Negredo penalty shout the only action of note.
Downing and Gibson struck strikes wide, before Barragan – already booked for a foul on Success – was lucky to avoid a red card after scything down the Nigerian attacker a second time.
It was a clear yellow card, but the referee decided against sending off the Spanish defender just half an hour in.
Negredo – a constant menace to the Watford defence – recorded the first shot on target four minutes before the break, but his weak strike on the turn was right at Gomes in goal.
The half ended with that as the only test either goalkeeper had to face, but both teams had a right to feel aggrieved at the standard of the refereeing.
The second period started in the manner the first ended; both teams were breaking well, but a lack of quality in the final third was seriously bringing down the game.
Forshaw managed a long-range attempt easily saved, Stuani flashed wide from just outside the area, and Success went down in the area – with nothing given.
But in the same move in which the winger went down, the Hornets finally took the lead – with their first shot on target.
The ball was cleared after Success went down, finding its way to Holebas on the left just outside the area.
The Greek wing-back charged forward, before firing off a shot which curled away from Valdes into the top left-hand corner of the net.
It was 1-0 to the visitors, and the goal appeared to calm Mazzarri’s men, who started passing the ball round in a much more composed manner than seen earlier in the game.
Boro were still only recording half-chances, with Gomes never properly tested, and Watford managed the next clear chance, with Success firing a powerful strike held well by Valdes.
A number of hopeful crosses were sent into the Hornets’ penalty area, but Gomes did more than enough to deal with everything he faced, securing a hard-fought win for the Hertfordshire side.