Wales midfielder King scored the only goal of the game at struggling Sheffield United to ensure boss Sven-Goran Eriksson's target of achieving automatic promotion might not be as optimistic as it first sounded.
Defeat was harsh on the Blades, who were on top for the majority of the game but were once again made to pay for some poor finishing, with Foxes boss Chris Weale also in outstanding form.
United boss Micky Adams is still yet to taste victory since being appointed in December but he will take heart from this performance against his former club.
Leicester stunned the Bramall Lane faithful by open the scoring just three minutes in when Ayegbeni Yakubu slipped but prodded the ball into the path of the onrushing King, who lifted the ball over Steve Simonsen with a neat left-foot finish.
The Blades responded and skipper Nick Montgomerie must have thought his venomous long-range drive was heading for the top corner until Weale intervened with a fine fingertip save.
There was a blow for United 28 minutes in when former Foxes left-back Joe Mattock appeared to get his left foot stuck in the turf and went down in agony.
Mattock, who was given a rough ride by the travelling supporters before sustaining the injury and while he was receiving treatment, was carried off with what appeared to be a serious injury.
The home side could count themselves unfortunate not to be level when midfielder Lee Williamson's measured free-kick came back off the post five minutes before half-time.
Elian Parrino also came close just before the break as United pressed for an equaliser.
Marcus Bent missed a golden opportunity to score his first goal since rejoining the Blades nine minutes into the second half, with the much-travelled striker blazing over the crossbar from just six yards out.
Leicester were riding their luck and the Blades felt they should have been awarded a penalty when the ball appeared to strike Yuki Abe's arm but the referee waved play on.
United remained on the front foot and the unmarked Shane Lowry rose to meet a Mark Yeates corner but headed wastefully over the bar from eight yards out.
It was all one-way traffic and Lowry fired over the bar when the ball fell nicely for him 12 minutes from time.
Parrino had a header cleared off the line a minute from time and Weale made an outstanding defender to deny Parrino once again in stoppage time as the Foxes somehow hung on for all three points.