But deep into injury time King pounced after Kari Arnason had blocked substitute Dany N'Guessan's shot.
Leicester made two changes with Yann Kermorgant and Paul Gallagher coming in for the suspended Matt Oakley and N'Guessan, who dropped to the bench.
The international break had afforded King time to recover from an arm injury.
Plymouth were boosted by the return of skipper Carl Fletcher in central midfield while Alan Judge overcame a knee injury to start and Craig Noone's foot problem had recovered sufficiently for him to figure on the bench.
Leicester took the early initiative with Kermorgant and Matt Fryatt firing wide.
Plymouth had to wait 20 minutes for their first attempt on goal as Alan Gow's free-kick was blocked by Martyn Waghorn.
Leicester's Richie Wellens let fly with a shot from 20 yards but Romain Larrieu fended it away from danger.
The Plymouth goalkeeper was again forced into action a few minutes later, this time denying Bruno Berner a goal on his 32nd birthday.
Kermorgant was determined to take his chance but again fired over the bar when the chance came.
Waghorn also shot over the bar before heading straight at Larrieu from Berner's corner following the restart.
The striker continued to take the fight to the visitors but again failed to find the target with a 25-yard effort.
Despite a number of long-range efforts, Leicester were lacking the incisive football to create clear-cut chances.
Waghorn's strike partner Fryatt was suffering similar frustrations and when his shot did appear to be on target, Shane Lowry was there to block for Plymouth.
With less than 20 minutes remaining, Leicester manager Nigel Pearson looked to his bench with N'Guessan replacing Kermorgant.
N'Guessan was presented an immediate chance from Gallagher's corner but headed wide.
Gow had a rare 25-yard effort for Plymouth but it was little more than a warning shot to Leicester over committing.
The Foxes increasingly ran short on ideas to break down their compact opponents and the game looked to be dying out with little fuss.
But their was late drama as King was finally presented with the chance and he took it to give the home side all three points.