Midfielder King produced a fine strike in the 63rd minute after collecting possession on halfway, driving at the heart of the Preston defence and smashing the ball into the net.
The Foxes began promisingly and went close in the third minute when Lloyd Dyer collected possession and hit a fierce shot which drew a fine block from defender Elliott Ward.
Four minutes later, Preston's on-loan midfielder Matt James engineered the space to drive a 20-yard shot over the crossbar.
However, Leicester continued to probe and in the 17th minute former Liverpool defender Jack Hobbs advanced forward and struck a shot from 35 yards which Keith Treacy did well to block.
North End winger Ross Wallace was then booked for unsporting behaviour and Leicester carved out a clear-cut opening shortly before the half-hour mark.
On-loan striker James Vaughan found himself unmarked six yards from goal but he could only mishit a poor shot wide of the target.
The Foxes were then dealt a blow in the 36th minute when Michael Morrison sustained an injury and was replaced by Nolberto Solano.
Yet Preston were also affected by injury at half-time as goalkeeper Andy Lonergan, who has enjoyed another fine campaign at his hometown club, was forced off and replaced by Wayne Henderson.
Preston were almost caught cold two minutes after the restart when Paul Gallagher crossed dangerously for Martyn Waghorn to hit a right-foot shot from six yards which flew over the crossbar.
The pendulum continued to swing to and fro and Preston goalkeeper Chris Weale did well to punch a deep cross from Billy Jones away from danger in the 54th minute.
Three minutes later, King sent a dipping volley just wide from 20 yards but he made no mistake in the 63rd minute.
It was a fine effort which saw him take matters into his own hands by picking up possession, advancing forward at pace and then seeing his progress halted on the edge of the Preston 18-yard box.
However, he quickly got to his feet and struck a fierce right-foot shot into the net to give the Foxes the vital breakthrough.
Visiting boss Nigel Pearson then withdrew Lloyd Dyer and replaced him with Nicky Adams in a tactical switch as Leicester sought to soak up the pressure.
Yet the Foxes had the chance to extend their lead as Waghorn lobbed narrowly over the bar from 12 yards out.