Vakoun Bayo's goal at the end of the game was the difference in a victory for the Hornets and a loss for the Canaries.
Bayo tapped the ball home after a slick move, which saw Ismaila sarr racing away down the left and showing great composure to pick out the strikers to net.
Keinan Davis twice went close before the break as the Hornets were the more threatening team.
But the Canaries, in their second game under Allan Russell, improved in the latter stages and twice threatened through Adam Idah, only for Bako's effort to ensure the Hornets move up to fourth in the Championship table.
The win was the perfect response from an injury-hitWatford side thrashed bySwansea City last time out, but a real blow to the hosts, who have failed to win either of their games since Dean Smith's dismissal.
With the stern words of boss Slaven Bilic still ringing in their ears after their hammering by the Swans, the Hornets showed much more unity in the early stages.
The on-loan Villa man's nifty footwork led to him having an early strike beaten away at the near post by the keeper, and he also sent a delicious curling effort narrowly wide before then squaring a pass to his teammate.
Grant Hanley's first real moment of note 10 minutes before the break was ruled out by the referee for being in an area that was not in line with the rules.
Josh Sargent, who scored three times in his previous three games against the Hornets, got free on the left but his cross was stopped by the keeper.
Big chances were rare after the interval.
Bayo missed a glorious one however, fluffing his lines horrendously five minutes after the restart when he failed to hit the target from point blank range.
The Canaries looked to be finishing the stronger, with Nunez dragging a shot wide from the edge of the box and Sargent lifting an effort over the bar.
Adam Idah, who missed the target when well placed and then curled a superb 20-yard strike a fraction wide, was their best chance.
The Canaries almost snatched a point when Gabriel Sara's free-kick grazed the bar in the ninth minute of added time.
Allan Russell told the radio station.
It was a good game. A switch off in focus or a poor decision can be fatal in these situations.
They took advantage of the counter- attack we gave them. We had a lot of chances to win it. The quality could have been better, but the effort was still there.
The standards have not been up to par. The players are willing to work hard. The dressing room is honest. It's a dressing that needs a boost.
Motivational issues are not an issue. The players have motivation, drive, and bravery. At times, we lacked quality.
Slaven Bilic spoke to the radio.
It was a good result. It was a great performance, even though it was difficult. Sometimes your team is asked to give more than they are capable of.
We had the better chances. It was a great performance. I am happy for the team. We have many injuries. We are not lucky.
The fans heard us. We had a good day. Normally you play with your second team in the FA Cup but we don't have a second team.
Some of the players will return very soon. Even when the situation is impossible, it's possible. Giving your maximum and being positive are the only ways to get something. It's three points, but it feels more than that.
In the transfer window, we have to be active. We have been active and have a couple of players.