On loan from Nottingham Forest, he gave the visitors the lead when he curled home a right-footed shot into the far corner of the net after seven minutes.
Darius Henderson doubled Watford's lead on 52 minutes when he rose above Darren Purse to head home.
King sealed the win on 67 minutes with a deflected shot before Cameron Jerome reduced the deficit late on.
Watford manager Adrian Boothroyd:
"We started brightly and scored at the right times. I'm very happy with my front players and feel that we don't rely on just one man.
"Cardiff will be a good side this season but that makes the performance all the better for us.
"I have some hungry players at this club and it looks good for the future as there are goals in my side."
Cardiff: Alexander, Darlington, Purse, Cox (Loovens 45), Barker, Cooper, Whitley, Mulryne (Koumas 58), Ardley, Jerome, Parry (Lee 58).
Subs not used: Weston, Margetson.
Goals: Jerome 80.
Watford: Foster, Chambers, Carlisle, DeMerit, Stewart, Devlin (Bangura 87), Mahon, Blizzard, Young (McNamee 74), King, Henderson.
Subs not used: Chamberlain, Bouazza, Osborne.
Goals: King 7, Henderson 52, King 67.
Ref: U Rennie (S Yorkshire).