Two goals in the final 12 minutes of the match saw the visitors claw their way back from 2-1 down to snatch the victory, and set up a final showdown with Charlton on Saturday.
City started brightly, and took a 15th minute lead when George Taft headed Michael Cain’s free kick home, only to see the visitors draw level after an unfortunate own goal from Sean McGinty.
After the break Leicester rallied and once again edged ahead when substitute Joe Dodoo found the bottom corner from 20 yards, but Cardiff midfielder Deji Oshilaja netted twice in the closing stages to seal a place in the national final.
Afterwards Development Squad Manager Steve Beaglehole told LCFC.com afterwards: “I’ve said to the lads that they’ve been fantastic this season, better than I would have anticipated and been very consistent.
“However I don’t think they’ve given a true account of what they’re capable of tonight. One of the things we have done this season is score goals and create chances in every game we’ve played, and we didn’t create enough tonight, although we scored two goals.
“I think overall on the night I’ve got to be honest and say I thought Cardiff deserved to win.”
It was an even opening to the match with Bluebirds striker Rhys Healey seeing an effort blocked by Taft early on, before The Foxes were able to take the lead from a set piece.
Jacob Blyth was challenged unfairly for an aerial ball on the left in the 15th minute, allowing Cain to find the head of Taft with a free kick to the far post, which the England youth international powered past visiting ‘keeper Elliot Parish with his head.
With a lively looking front three working hard, Cardiff almost responded through Etien Velikonja just minutes later, but his left foot volley flew over the bar, while at the other end Tom Hopper did the same on city’s left flank.
The game continued to swing from end-to-end, and in the 28th minute a huge slice of fortune saw Cardiff draw back on level terms.
Left back Declan John advanced down the flank and fired in an excellent ball for Velikonja, only for trialist McGinty to head past Foxes ‘keeper Adam Smith at the near post and give the visitors a goal.
Leicester responded well, and had Luke Hendrie’s left footed effort not struck the post after a neat turn in the area, Beaglehole’s men would have taken a 2-1 lead into the break.
Again after the restart for the second 45, City started the stronger with Blyth testing Parish, but were unable to make that second breakthrough until the introduction of Dodoo for Harry Panayiotou changed things in the host’s favour once more.
Just after the hour mark, the England youth international worked the ball onto his right foot on the edge of the box and rifled a low shot expertly into the bottom left corner of the goal for 2-1.
City looked to be comfortable, without creating a great deal of openings to extend their lead, but on 78 minutes a corner was driven into a crowded penalty area and took a deflection off Oshilaja to take it past Adam Smith.
Then, with extra time looming, the same player struck again to put the game beyond reach, but on this occasion it was a sweetly struck right footed strike from the edge of the box after a clever turn had engineered the opening.
Jamie Anton did manage to tee Tom Hopper up for a late effort, but it wasn’t to be for The Foxes, who can now look forward to next season.
City (4-3-3): A. Smith; Anton, McGinty, Sesay (c), Taft; Clifford (McCourt 84), Hendrie, Cain; Blyth, Hopper, Panayiotou (Dodoo 58)
Unused subs: Maduako, J. Smith, Paratore
Goals: Taft 15, Dodoo 63
Cardiff (4-3-3): Parish; Nugent, John, Yorwerth, Oshilaja; Kiss, Wharton, Ralls (c); Velikonja, Healey, Harris
Subs: Richards, Owen, O’Sullivan, Darko, Bell
Goals:McGinty og 28, Oshilaja 78, 86