Adlene Guedioura scored in either half – including a 30-yard strike – as the Hornets inflicted a heavy defeat on the Bluebirds and took a foothold in the Sky Bet Championship play-off places. Odion Ighalo and Gabriele Angella also netted in a third successive away win.
Cardiff led through Adam Le Fondre’s early strike but were outplayed by the visitors and only an injury-time header from Kenwyne Jones saved manager Russell Slade from his heaviest defeat since taking charge.
However, the result and performance heaped more pressure on Slade after a fifth game without a win. Their last victory was last month at Watford.
Cardiff’s edged the opening 40 minutes but their only genuine threat appeared to be from set pieces and the boot of Peter Whittingham.
Tom Adeyemi should have handed them the lead as early as the seventh minute but failed to connect inside the six-yard box when Sean Morrison turned in Whittingham’s free-kick.
Whittingham then fired just wide of Heurelho Gomes’ left post from in the 14th minute after Craig Noone was felled.
It was the veteran midfielder’s dead-ball expertise that provided the opening goal. Le Fondre won the free-kick and then touched Whittingham’s curling delivery past Gomes on 20 minutes.
However, Watford steadily played their way back into the game despite having top scorers Troy Deeney and Matej Vydra on the bench, turning the game on its head in the space of five minutes before the break.
Juan Carlos Paredes and Ighalo combined, only for Fernando Forestieri to shoot tamely straight at Bluebirds goalkeeper David Marshall from point-blank range.
However, Paredes quickly fired the ball back into the danger area and Guedioura flicked his effort past Marshall for his first goal of the season.
Daniel Pudil then provided a looping left-foot cross and Ighalo leapt highest between centre-backs Bruno Manga and Morrison to score at the near post.
It was a telling blow as Cardiff’s hustle and bustle wilted against a team who simply passed the ball quicker and found space easier to come by.
Ighalo and Guedioura both went close to scoring a second goal, while Forestieri had an attempt from 25 yards out that Marshall only controlled at the second attempt.
However, there was little Marshal could do when Guedioura unleashed a wonderful strike from 30 yards into the top corner to put Watford 3-1 up in the 62nd minute.
Things got even better for the visitors when Deeney stepped off the bench to hand Angella an easy header in to put the visitors out of sight.
Although Noone did the same for Jones in added time, it was too little too late for Cardiff.