The former Carlisle United striker was in the right place to convert from close range nine minutes after the restart when the Tykes fell asleep at a Hornets set-piece, allowing the unmarked John Eustace to head Don Cowie's free-kick back across goal.
Having sold key and experienced players like Tamas Priskin, Tommy Smith, Jobi McAnuff and Mike Williamson in the transfer window, the last thing Hornets boss Malky Mackay needed in the build-up to this game was to lose one of his few remaining old heads through injury.
But Jay DeMerit was sidelined after sustaining a groin problem while away on international duty with the USA, so Dale Bennett was pitched into the heart of the defence for his senior debut.
However, the 19-year-old acquitted himself admirably in helping Watford to their first league clean sheet of the campaign, but it was going forward where two other youngsters caught the eye.
Henri Lansbury and Tom Cleverley, on loan from Arsenal and Manchester United respectively, caused Barnsley problems throughout with their control, movement and speed of thought, although it was a quickly taken free-kick from Cowie that nearly set up Graham for the opener, but he shot straight at David Preece early on.
Despite Watford's dominance, this was to be the only time the Barnsley keeper was tested in his first half, but he was rooted to his line when Watford scored after 54 minutes.
The only criticism that could be levelled at the home side was their failure to score a second made the closing stages unnecessarily nervous.
It took a superb tackle from Lloyd Doyley to prevent Andy Gray from getting on the end of a cross-shot and having a clear chance to snatch an undeserved equaliser.
But Watford twice nearly scored again in the closing stages with Cleverley curling a 25-yard effort narrowly wide of the far post before substitute Nathan Ellington saw a rising effort clear Preece but rebound off the face of his crossbar.