Blues more than matched their opponents with a display full of pride and passion, and looked set for extra-time after Jo Kuffour’s stunning first-half strike, but St Ledger had the final say, heading home from close-range after Matt Ingram had kept out an initial effort.
Gareth Ainsworth made four changes with Steven Craig coming in for his debut, while Kortney Hause, Matt Spring and Jo Kuffour started in place of Jon-Paul Pittman, Leon Johnson, Paris Cowan-Hall and Sam Wood.
Dean Morgan crashed an early free-kick over the bar and Spring then saw an effort deflected as the Blues started brightly.
Matty James dragged a 20-yard strike wide before Marvin McCoy’s dangerous cross from the right just evaded Kuffour and Josh Scowen.
The visitors took the lead in the 13th minute when David Nugent converted from the spot after Stuart Lewis had tripped Andy King in the area.
Morgan was denied a fine equaliser in the 16th minute when Kasper Schmeichel did well to get down and parry away the forward’s low drive after the 29-year-old had linked up well with Craig on the right.
Parity was restored in the 20th and in some style! Kuffour received the ball with his back to goal after a neat spell of passing, turned his marker and crashed home an unstoppable effort into the top corner.
The two sides cancelled each other out for long spells after until 10 minutes from the break when Morgan’s flick-on just eluded Craig.
Charles Dunne’s 30-yard free-kick forced Schmeichel into a save five minutes later, but it was the visitors who had the final chance of an even first half when Wes Morgan headed James’ corner over from eight yards.
Scowen shot straight at Schmeichel 10 minutes after the restart, but Ingram came to the rescue 60 seconds later, producing a stunning save low to his right to keep out Neil Danns’ powerful drive.
Ingram made another stunning save on the hour, this time tipping over Dyer’s goalbound strike after the wing-back had been found by substitute Anthony Knockaert.
Wood flashed a shot wide from a narrow angle, but the Wanderers were continuing to frustrate the Foxes and they had a chance of their own when skipper Lewis shot over the bar.
The visitors continued to press for a late winner, but Ainsworth’s men threw their bodies on line to protect Ingram’s goal.
Max Kretzschmar was introduced two minutes from the end of normal time and a minute later Stewart headed Scowen’s corner wide.
Ainsworth made his final change as the game entered injury-time with Cowan-Hall replacing Kuffour, who left to a standing ovation from the Adams Park faithful.
However, there was still time for some late drama as St Ledger headed in from close-range after Ingram had done well to parry an initial effort.
Blues: Ingram, McCoy, Dunne, Stewart, Hause, Spring, Lewis (c), Scowen, Morgan (Kretzschmar 88), Kuffour (Cowan-Hall 90), Craig (Pittman 75).
Subs: Horlock, Wood, Arnold, Fletcher.
Leicester: Schmeichel, Moore, De Laet, St Ledger, Morgan (c), Danns, James (Knockaert 51), King, Dyer, Nugent (Schlupp 61), Wood.
Subs: Logan, Waghorn, Gallagher, Hopper, Bakayogo.
Attendance: 3,158 (1,140 Leicester)