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.
9. Samuel Bastien 6
Broke play up well early on, but Watford's physicality became a problem. Photo: Ashley Allen
10. Ashley Barnes 5.5
Another to be on the end of some rough stuff, with his cameo here late last season fresh in the memory. Photo: CameraSport - Rob Newell
11. Dara Costelloe 5.5
Worked hard and made some good runs in behind, but handled well by Gaspar. Replaced at the break by Cork. Photo: OLI SCARFF
12. Jack Cork 6.5
Steadied the midfield following his introduction at the break and played a wonderful pass into Vitinho that led to Kamara's red card. Photo: Ashley Allen