King shrugged off his marker on the stroke of half-time and found half a yard just inside the box before shooting low into the left corner.
But the on-loan striker should have won the game for high-flying Hornets after missing a second-half penalty.
Brighton levelled when Seb Carole's free-kick was met by Guy Butters, whose flicked header looped into the net.
Brighton boss Mark McGhee:
"I think we've been playing well but the 13 draws we've had now don't help. Eight or nine of them we could and should have won.
"I'm getting everything from those I have but we're a couple of lads short.
"We need a player to turn the 13 draws into six wins and then we would be in the top half."
Watford: Foster, Chambers, Mackay, Carlisle, Stewart, Devlin, Mahon, Bangura, McNamee, King, Grant (DeMerit 69).
Subs Not Used: Chamberlain, Diagouraga, Mariappa, Gill.
Goals: King 45.
Brighton: Blayney, Reid, Butters, Frutos, El-Abd, Hart, Oatway, Hammond, Carole (Nicolas 82), Robinson (Knight 62), Kazim-Richards (McPhee 63).
Subs Not Used: Mayo, Chaigneau.
Booked: Reid, Hart.
Goals: Butters 51.
Ref: R Olivier (W Midlands).