In a turgid start by both sides, the game was 24 minutes old before it saw a meaningful attempt on goal when Matt Holland fired a half cleared corner wide.
City squandered their first serious assault of the contest on 33 minutes when Jonathan Hayes played in Iain Hume, whose sliced shot was driven into the side-netting by Richard Stearman.
Minutes before the break, Patrick Kisnorbo headed a Clemence corner over before in time added on, City went their closest yet when Hume's clever pass found Matt Fryatt whose angled shot beyond Nicky Weaver had to be cleared off the line by Madjid Bougherra.
Five minutes after the restart, Fryatt returned the favour when he squared the ball to Hume, whose deflected shot forced the Charlton keeper into a diving save.
Weaver was then forced to punch away Alan Sheehan's free-kick as Leicester at last began to build up a head of steam.
Charlton eased the pressure with a flowing breakaway which deserved better than Luke Varney's inaccurate shot.
Fryatt then escaped the attention of two Charlton defenders to squeeze in a shot which had Weaver at full stretch, while at the other end Marton Fulop had to be equally alert to block Varney's fierce drive with his legs.
But it was Leicester that broke through on 78 minutes when Stearman crossed from the right and Clemence stooped in front of Yassin Moutaouakil to score with a header which crept beyond Weaver's despairing dive.
But then at the death, McCarthy stunned the home crowd with an equalising goal.
Chris Powell launched a long free-kick into City's penalty area which was only half cleared to McCarthy, who hooked the ball over his shoulder and into the net.