Phil Brown's City were predictably organised and hard to break down, but once King had made the breakthrough after 20 minutes they were always up against it because of their lack of a cutting edge in the final third of the field.
Despite making an unusually sluggish start Watford took the lead in fine fashion. Spotting King's run to the right of the penalty area, captain Gavin Mahon played a superb 30-yard pass to the striker, who easily bundled his way past Damien Delaney before striking a low, angled drive through Wayne Brown's legs, past Boaz Myhill's dive and in-off the far post.
Hull's task could have become harder seven minutes later when David Livermore, who was one of three City players to be booked in the first half, was fortunate to escape a second caution after appearing to jump late into full debutant John-Joe O'Toole.
The Hornets were always in control until the break, but found chances difficult to come by against the disciplined Hull rearguard.
The closest they came to a second was when Adam Johnson's back-heel found King, whose ball across the penalty area seemed destined to give O'Toole a straightforward opportunity until Richard Garcia nicked it away from him at the last minute.
Watford almost became the architects of their own downfall after the break by defending too deep at times, but the closest Hull came to equalising was when Dean Marney's superb ball picked out Bryan Hughes' run from left to right and in behind the home side's defence, but the former Birmingham midfielder was unable to lift the ball over Richard Lee.
The Hornets' afternoon would have been more comfortable had Dan Shittu not blazed a clear-cut chance high and wide of the near post after being picked out by a Johnson corner, but in the end they got over the finishing line without being too unduly troubled.