Chances were few and far between in the opening exchanges, with a cagey 45 minutes in the first half seeing Caglar Soyuncu go closest with a header over the Chelsea crossbar.
An early injury to Jonny Evans did little to dampen the Leicester spirits, and midfield marshal Youri Tielemans unleashed an unstoppable effort into the top corner to break the deadlock in the 63rd minute.
The ball did appear to strike Ayoze Perez's arm in the build-up to Leicester's opener, but Chelsea's appeals were waved away as they went in search of an equaliser, with Ben Chilwell so nearly restoring parity had it not been for Kasper Schmeichel's heroic save from his header.
The Blues thought they had levelled the scores in the 89th minute as an episode of pinball in the Leicester area saw Wes Morgan turn into his own net, but Chilwell was deemed to be offside in the build up.
Leicester's staunch defence held on for the final few minutes as Brendan Rodgers led the Foxes to their first-ever win in the prestigious competition, with Chelsea suffering back-to-back defeats in the final following last year's defeat to Arsenal.
The two finalists now prepare to renew hostilities in Tuesday's Premier League fixture at Stamford Bridge, with both seeking to be assured of a place in the top four of the table.
6th min: Chelsea's early dominance leads to a corner which Reece James whips in, but Leicester - under the cosh - manage to clear.
15th min: Antonio Rudiger lets fly from long range, although his wide effort was never troubling Kasper Schmeichel.
20th min: Leicester free kick in a promising area on the right after Thiago Silva's handball - Youri Tielemans takes and Caglar Soyuncu heads over the bar.
29th min: Silva so nearly picks out Cesar Azpilicueta - totally unmarked - at the back stick, but the smallest of touches from Werner takes it away from his teammate.
33rd min: Jonny Evans's race is run as the Leicester centre-back - reinstated to the first XI - is forced off through injury. Marc Albrighton comes on.
42nd min: Soyuncu gets his head on another Leicester free kick from Luke Thomas, but the Turkish defender cannot get his effort on target.
44th min: Albrighton loses the ball and Werner's effort takes a nick off Wesley Fofana and out for a corner, which comes to nothing.
45th min: Vardy has a late chance to break the deadlock from Youri Tielemans's ball into the box, but James does enough to put his compatriot off.
53rd min: Kante breaks down the right and Marcos Alonso manages to meet the Frenchman's cross - his header is far too weak to trouble Schmeichel, though.
62nd min: Azpilicueta's attempted cross ends up finding its way to Schmeichel's near post, and the Leicester keeper calmly gathers down low.
63rd min: Leicester open the scoring in some style. Ayoze Perez intercepts James's attempted pass and Thomas picks out Tielemans, who unleashes an unstoppable effort into the top corner from outside the area. Chelsea's appeals for a handball prove futile. 1-0 Leicester.
78th min: Former Leicester man Ben Chilwell so nearly equalises against his former club, but Schmeichel gets a vital hand on his header to claw it away from danger.
83rd min: Rudiger attempts to replicate Tielemans's spectacular long-range effort, but the defender's shot is far too high.
87th min: Another potentially game-defining moment from Schmeichel, who gets a strong hand to Mount's goal bound effort to turn it away for a corner.
89th min: Leicester's resilience is so nearly broken. Silva picks out Chilwell down the left and a pinball between Soyuncu and Wes Morgan ends with the ball ending up in the back of the net, but VAR rules it out for the tightest of offsides.
MAN OF THE MATCH - YOURI TIELEMANS
Wesley Fofana and the heroic Schmeichel deserve an honourable mention in the Leicester defence, but there was only ever one candidate for this man of the match award.
Even before his second-half thunderbolt, Tielemans ran the show in midfield and demonstrated his quality throughout the 90 minutes with a confident and assured display - what a signing he has turned out to be for Leicester.
A rather tame FA Cup final exploded into life with one of the great goals that the prestigious tournament has seen, as Youri Tielemans unleashed an effort that no goalkeeper in the world could have stopped.
There was an air of controversy surrounding the goal as the ball appeared to take a nick off the arm of Ayoze Perez, but the Leicester fans in attendance did not care one bit as jubilant scenes followed for the Foxes.
1963 - Youri Tielemans is the first Leicester City player to score in an FA Cup final since Ken Keyworth in 1963 vs Manchester United. Rocket. #FACupFinal— OptaJoe (@OptaJoe) May 15, 2021
Possession: Leicester 36%-64% Chelsea
Shots: Leicester 6-13 Chelsea
Shots on target: Leicester 1-3 Chelsea
Corners: Leicester 4-10 Chelsea
Fouls: Leicester 9-13 Chelsea
LEICESTER CITY (3-4-1-2): Schmeichel; Fofana, Evans (Albrighton 33'), Soyuncu; Castagne, Ndidi, Tielemans, Thomas (Morgan 82'); Perez (Choudhury 82'); Iheanacho (Maddison 67'), Vardy
CHELSEA (3-4-2-1): Kepa; James, Silva, Rudiger; Azplilicueta (Hudson-Odoi 76'), Jorginho (Havertz 75'), Kante, Alonso (Chilwell 68'); Ziyech (Pulisic 68'), Mount; Werner (Giroud 82')