Tom Davies put the hosts ahead early on but Watford equalised before the break through King from close range.
King, who didn’t score for Everton during his spell there in the first half of 2021, then struck twice inside six minutes before Emmanuel Dennis put the seal on an incredible comeback.
It is a second successive home defeat for Everton’s former Liverpool manager Rafael Benitez and some boos greeted the Spaniard’s decision to replace Anthony Gordon with Richarlison in the second half.
He seemed vindicated when the Brazilian struck with a fine diving header just three minutes after coming on, but Benitez then saw his side’s lead evaporate in a disastrous final 10 minutes that leaves his side in eighth place.
Hornets sting tentative Toffees
When Davies slid in to convert Demarai Gray’s low cross from close range after three minutes it looked like being another difficult day against Merseyside opposition for Ranieri following their 5-0 thrashing at Vicarage Road a week earlier.
But while Everton failed to build on that start and were tentative, Watford took the attack to the home side in defiance of having the lowest number of shots in the Premier League – 76 – prior to this match.
King’s equaliser was initially ruled out for offside by referee Graham Scott but after looking at the pitchside monitor he overturned his decision because Gray had just played King onside.
Watford rallied after Richarlison’s goal – with Ranieri immediately throwing on Dennis and Joao Pedro – and Kucka got the visitors back on level terms with a powerful header from Cucho Hernandez’s corner.
Ranieri’s substitutions then sparked the late drama, with Dennis’ cross finding King with the time and space to steady himself for his second before the pair linked up again when the Norway striker kept his head again inside the box to claim his hat-trick.
Dennis has enjoyed a good start to his Premier League career, despite Watford’s struggles, and the Nigerian claimed his third goal of the season after an assist from Pedro as large sections of the home support headed for the exits.