Lee Holmes put Preston ahead seven minutes before the break and Nicky Wroe doubled the advantage after 78 minutes after pouncing on goalkeeper Eirik Holmen Johansen's error.
And King wrapped up the win in stoppage time when he headed home to make it five home games unbeaten to leave the visitors languishing in the drop zone.
The home side took the lead when Holmes slotted under Johansen after a neat cutback from Stuart Beavon.
Beavon looked lively on his return and made space for a 20-yard effort shortly after the break but it never troubled Johansen.
Scunthorpe's Akpo Sodje wasted a chance on the hour, Declan Rudd missing a looping cross but the former Preston striker sliced his shot off target.
And the Lilywhites doubled their lead with Wroe firing into an empty net from 40 yards after Johansen botched his attempted clearance.
King saw a shot hit a post in the dying minutes in a goalmouth scramble but he got on the scoresheet in stoppage time, nodding in Jeffrey Monakana's cross to seal the win.