King, who left Hull after falling out with manager Phil Brown in January, scored with two minutes remaining to secure a crucial victory at the Riverside Stadium.
Tuncay Sanli and Matthew Bates were also on target for Boro, either side of a Manucho equaliser.
With manager Gareth Southgate sensing King might have a point to prove, the on-loan Wigan forward was drafted into an attacking line-up to complement Tuncay, Afonso Alves and Jeremie Aliadiere.
Despite having the least prolific attack in the Premier League and his side being five points adrift of safety, Southgate's intentions were clear from the outset.
The bold approach made for a fast and open game with both sides committed to throwing men forward.
Boro reaped the rewards after just three minutes as the much-maligned Alves won the ball in midfield and exchanged passes with King before racing unchallenged into the box.
Matt Duke saved low to his right as Alves shot across him but, with the Hull defence absent, Tuncay was first to the rebound and coolly clipped into the net.
The hosts looked confident and Brazilian Alves was then denied as Duke came to meet him at the edge of the area.
Yet Boro's lead lasted just six minutes as Hull, also feeling the heat after just one win in 15, produced the perfect response.
Geovanni won a corner with a shot that was narrowly deflected over and the ball ran wide to Nick Barmby from the resulting set-piece.
Former Boro midfielder Barmby lifted the ball back into the box and on-loan striker Manucho rose highest to head in his second goal for the Tigers.
Bernard Mendy threatened a second moments later after racing onto another fine Barmby ball but Brad Jones bravely met him on the 18-yard line.
Alves then fired over after King brought down a long ball from Tony McMahon.
Chances continued to come at either end with David Wheater heading over from a Stewart Downing free-kick.
Craig Fagan then broke clear at the other end after a good Barmby pass but Jones just did enough to deny him a shooting chance. The ball broke to Manucho but the onrushing Angola striker scooped wide from a tight angle.
Boro reclaimed the lead just before the half-hour as Hull were caught out at a corner.
The visitors had no-one at the near post to clear McMahon's low cross and local product Bates nipped in to stab home his first Boro goal from six yards.
Robert Huth had the first chance of the second half when he headed across goal from another Downing free-kick.
Another opportunity seemed to open up for King but his mishit shot was deflected away from goal.
Downing, captain in the absence of the injured Emanuel Pogatetz, then curled a free-kick narrowly wide after Aliadiere was fouled on the edge of the area.
Hull's hopes were hampered by poor delivery into the box but Wheater needed to head off the line after Jones spilled a Sam Ricketts cross under pressure from Manucho.
Boateng, after three months out, entered the fray against his former club with just under 20 minutes remaining and soon fired over from distance.
Aliadiere appealed for a penalty after a challenge from Kamil Zayatte but referee Phil Dowd was not interested.
Downing had a great chance when he cut back inside the area but Duke just managed to get a hand to his shot inside the post.
Ricketts then picked out Dean Marney with a good cross but his header was deflected over.
Manucho also headed over from an Andy Dawson cross as time began to run out for Hull.
Boro killed off the game when Aliadiere robbed Boateng with two minutes remaining and King rifled past Duke.
Boateng could easily have been sent off for trying to hamper Aliadiere but Boro did not complain as King powered forward to seal only a second win in 20 games.
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