The 31-year-old, who also scored in the Addicks 2-1 victory in the reverse fixture back in August, was playing in Leicester for the first-time since his costly play-off semi-final penalty miss two years ago during an ill-fated spell with the Foxes.
Booed throughout, Kermorgant, almost inevitably, had the last laugh as Charlton performed a textbook smash-and-grab raid against a Leicester side who dominated from start to finish.
Leicester's first opening came in just the third minute as David Nugent advanced down the left wing and squared the ball for Anthony Knockaert, who rode a challenge on the edge of the area before delivering a fierce shot which Ben Hamer did well to parry behind.
The visitors went in front after 19 minutes when Pritchard got free down the left channel and pulled back for Kermorgant, who neatly converted from 12 yards to a chorus of boos from the fans of his former employers.
Marshall came close to restoring parity in the 37th minute when he collected a cross-field ball on halfway, surged forward and, as the Charlton defence parted, flashed a powerful drive which Hamer initially spilled before smothering.
Knockaert fluffed a golden opportunity to equalise six minutes before half-time. Richie DeLaet went on a marauding run down the right flank, beat two Athletic defenders and picked out the Frenchman, who ballooned his effort over the bar with only Hamer to beat.
Charlton showed little attacking intent throughout the evening, but they might have had a second four minutes after the interval when Kermorgant brilliantly put Pritchard clean through, only for the striker's audacious attempted lob over Kasper Schmeichel to sail harmlessly into the crowd.
The Danish keeper then spilt Johnnie Jackson's 57th-minute free-kick at the feet of Michael Morrison, only to recover superbly and tip the defender's shot away.
Wood's anger intensified seven minutes later when Leon Cort appeared to deflect his 20-yard shot behind with his arm, but it was a case of third time lucky just two minutes later.
DeLaet went on another foray into opposition territory and the Belgian's centre deflected kindly for the 21-year-old to fire home an eighth goal in seven matches since arriving from West Brom.
There appeared to be only one winner at that stage, but Charlton substitute Danny Haynes latched on to Kermorgant's flick and, with the Leicester defence standing off, unleashed a 20-yard left-foot effort which sailed into the top-left corner of Schmeichel's goal.