King's power and pace proved decisive as he drove home midway through the first half to enable Watford to move four points clear at the top.
Earlier Watford had a lucky escape on 14 minutes when Bryan Hughes' effort could only find the side-netting.
On-loan Manchester United striker Fraizer Campbell caught the eye for Hull and he went close with a shot after good work from Dean Windass.
Hull boss Phil Brown:
"One win in six is not acceptable with the squad of players we have here and some strong words have been said in the dressing room.
"I can't keep saying that we should have got a draw when we get beaten and we should have won when we draw.
"At least we have the chance to put things right on Monday night with a Yorkshire derby against Barnsley."
Watford: Lee, Doyley, DeMerit, Shittu, Stewart, Smith (McAnuff 73), O'Toole, Mahon, Johnson, King, Darius Henderson (Ellington 73).
Subs Not Used: Loach, Priskin, Mariappa.
Goals: King 20.
Hull: Myhill, Turner, Delaney, Ricketts (Elliott 80), Brown, Garcia (Windass 52), Marney, McPhee, Hughes, Livermore (Featherstone 78), Campbell.
Subs Not Used: Duke, Dawson.
Booked: Livermore, Brown, Marney.
Ref: Andy Woolmer (Northamptonshire).