Miguel Britos's first goal for the Hornets proved enough to condemn Sunderland to another defeat, and they remain seven points from safety.
Watford started to create the better of the chances, with Etienne Capoue sending a shot just wide from distance, before Pickford was required to turn behind Niang's rasping effort.
Craig Cathcart and Jose Holebas also had opportunities for Watford, but they finally made the breakthrough just short of the hour mark when Britos headed into the net after an earlier shot had hit the crossbar.
Minutes later, Isaac Success raced clear on goal before being denied by Pickford, before Janzuaj kept Gomes honest with a shot at the other end.
The chances continued to come thick and fast as Holebas and Success both tried to register a second for Watford, but Sunderland almost equalised when Gomes diverted a Borini effort wide of the goal.
For the first time in the match, the chances began to peter out as full time approached, and Watford were grateful to hang on for a win which sees them move into the top half of the standings.