The Robins went into the game having taken only four points from the last available 24, but they went some way to improving that record after just 23 seconds.
A poor header from Watford's top scorer Tommy Smith, who tried to be too clever near the edge of his own penalty area, give Ivan Sproule the chance to lift the ball back into the box and Nicky Maynard got first run on Jay DeMerit to steer his shot beyond Scott Loach.
The two City players combined again to double their side's advantage after 29 minutes. This time Sproule got the better of a poor challenge by Mat Sadler to fire in a cross-shot which Loach could only parry back into the heart of the area to present Maynard with a straightforward chance for his second.
But Watford were back in the contest four minutes after the restart when Jobi McAnuff's run down the right ended with a low cross which broke off a defender and into the path of Grzegorz Rasiak. He celebrated his return to the starting line-up after a ten-game absence with the third goal of his loan stint with the Hornets.
But the home side's joy was to be shortlived as, three minutes later, Marvin Elliott hit a 25-yard right-footed drive that, although it took a slight deflection off Lee Williamson, should not have gone straight through Loach in the manner which it did.
However, it was then Elliott's turn to feel embarrassed when he met Smith's right-sided cross with a diving header after 65 minutes, but only succeeded it in diverting it past Adriano Basso.
An equaliser looked on the cards for Watford at that point, but it never materialised and instead City made sure of the points in stoppage time when substitute Dele Adebola headed in Lee Johnson's cross.