The Clarets’ run of 25 Championship games without defeat — stretching back to late 2015 — came to an end at Vicarage Road.
The visitors once again bossed possession, owning 65% of the ball, but conceded the only goal of the game with what was Watford’s only shot on target.
Tom Cleverley passed the ball into the net right on the stroke of half-time after being picked out inside the penalty area by Ken Sema.
Josh Brownhill almost salvaged a point for the away side with the last kick off the game, but the midfielder’s volley from substitute Vitinho’s cross struck the crossbar.
Here are Chris Boden’s player ratings.
13. Jay Rodriguez 6
First appearance of the season when replacing Barnes from the bench just after the hour and Burnley carried a greater threat in the final third following his introduction. Photo: CameraSport - Rob Newell
14. Nathan Tella 6
Utilised on both flanks when handed his debut after the hour, but will ultimately need time to get into the swing of things having arrived at the club on loan just a day before the trip to Vicarage Road. Photo: Andy Ford/Burnley FC
15. Vitinho 6
Carries frightening pace, shows a burning desire to get forward, and forced Kamara into the error that led to his dismissal. Photo: Andy Ford/Burnley FC