The Lions were reduced to 10 men in the second half when Jake Cooper was given his marching orders, but they adapted to their numerical disadvantage.
Their resilience paid off in the 90th minute as defender Shaun Cummings scrambled home the winner to fire Millwall into the quarter-finals.
Find out how the action unfolded with our minute-by-minute text commentary below.
1.58pmGood afternoon and welcome to our live text coverage of the FA Cup fifth-round showdown between Premier League champions Leicester City and League One Millwall, a tie which has 'potential shock result' written all over it.
2.02pmThe Foxes are having a torrid time defending their unlikely Premier League crown and things have been particularly bleak for them of late, with no wins in six and five defeats on the bounce in the league.
2.04pmWe'll bring you more about the Foxes' recent misfortune shortly because team news is our next port of call...
2.08pmThe big news for the home side is that their danger man Lee Gregory has only been passed fit enough to start the afternoon on the bench. Steve Morison, who is Millwall's fifth all-time leading goalscorer, provides the threat up front.
2.11pmMeanwhile, in the Millwall defence, Byron Webster makes his 100th appearance for the club. His task will be keeping the likes of Kapustka and Okazaki quiet as the League One side go out in search of a cup upset.
2.14pmThey don't call Leicester boss Claudio Ranieri the Tinker Man for nothing - he's made wholesale changes this afternoon, with fringe players including Okazaki, Zieler and Gray all getting the nod. This leaves an embarrassment of riches on the away bench, with Vardy, Albrighton, Mahrez and Fuchs among the substitutes.
2.19pmThere's a good reason why this match has been earmarked as a potential giant killing. We've already touched on Leicester's relegation form, but take into account the fact that they've not won away since last April and have a Champions League clash with Seville to prepare for, and a shock seems even more likely.
2.22pmMeanwhile, Neil Harris's Millwall are on a good run in League One having gone unbeaten since last December and broken into the playoff positions. Ranieri has even hinted that his side are the underdogs for this one. That's the manager of the champions of England insinuating that his side are second favourite against third-tier opposition. Talk about reverse psychology!
2.27pmHere's what Ranieri had to say ahead of kickoff: "Our target is the Premier League - that is it. Of course we play in the FA Cup and Champions League and we want to do our best, but our aim is to be safe at the end. The goal is the Premier League. I want to give another opportunity to the players who played against Derby for them to show how good they are."
2.28pmAnd his opposite number, Neil Harris: "The team spirit is excellent and individually and collectively we are playing really well. This is a huge game for us, the players and the fans are really looking forward to it but I will admit that my mind has already turned to Chesterfield on Tuesday."
2.31pmThere's plenty of recent history between these two sides, even though they haven't spent a great deal of time in the same division. They've met four times in the FA Cup, each time at Millwall, and have two wins apiece.
2.33pmThey were in the Championship together in 2013-14 and Leicester defeated today's opponents at home and away, having after lost their previous four meetings with Millwall.
2.36pmAlthough Leicester have a good track record against third-tier sides - they haven't lost to one since 2001 - the Lions have lost only one of their last 10 FA Cup home matches. History doesn't appear to favour either team particularly but going off recent form, Millwall have every reason to fancy their chances today.
2.42pmAnd if they need any giant-killing inspiration, non-league Lincoln City defeated Premier League Burnley to book their place in the quarters of the FA Cup today. You can read all about the history-making Imps here.
2.45pmThere's 15 minutes to go until kickoff, which gives us ample time to look over both teams' route to the fifth round. Starting with Leicester, they overcame Everton away from home in round three before seeing off Championship Derby County 3-1 in a fourth-round replay.
2.49pmIt was a longer route to round five for Millwall, who entered the competition at the first round proper. They saw off Southend United in that fixture before hitting five past non-league Braintree in round two. They stunned Premier League Bournemouth by beating them 3-0 at home in round three before edging past Watford 1-0 in round four. Whatever happens today, that will go down as an impressive FA Cup run.
2.53pmPREDICTION: I'm going with a 2-1 win for Millwall today. Although Leicester are clearly the stronger side on paper, too much is going against them of late. The pressure on Ranieri is almost at critical mass, and with one eye on next week's Champions League fixture, I can see the Foxes slipping up here.
2.56pmOne of Millwall's celebrity supporters has wished them well today, but can they deliver a knockout blow to the Foxes? Stay tuned to find out!
1 minKICKOFF! Millwall, playing in white and blue stripes today, get the match underway. The hosts will kick from left to right in the first half.
2 minIt's a bright start from Leicester as Ahmed Musa plays a decent cross in from wide on the right. Demarai Gray strikes in on the half-volley but his effort bobbles harmlessly wide. Confident play from the visitors but Gray's effort never looked like posing Millwall any problems.
6 minMillwall attempt to move the ball along the right channel but Andy King is on hand to snuff out the danger. Leicester were keeping the ball well until that point but they have the chance to stage a counter after regaining possession. However, Gray's final ball to Musa is wanting and the move collapses.
9 minThe Foxes are looking watertight in midfielder. Millwall aren't sure whether to exploit the flanks or play the ball through the middle as the visitors have been quick onto everything in the centre of the park.
10 minGood inter-play between Morison and Onyedinma almost carves out a chance for Millwall before Mendy takes out the latter with a heavy - but fair - challenge. Leicester break after regaining the ball and go for goal through Gray, who curls a shot just wide of the top corner.
14 minPatient play from Leicester who maintain possession outside of the Millwall box, searching for a way through. The Lions are back in numbers and they restrict their opponents to a long-range attack, which results only in a goalkick as Musa slices an effort closer to the corner flag than the net. I suspect the state of the playing field was responsible for the quality of the Leicester man's finish there.
16 minCHANCES! A brilliant interception from Byron Webster keeps out Leicester as Musa finds plenty of space at the near post and gets a shot away. The Millwall defender does will to make himself big and turn the ball behind. Kapustka plays the ball back in and Okazaki turns it towards goal, forcing Lions goalkeeper Jordan Archer into an acrobatic save!
20 minLeicester are enjoying a purple patch at the moment. They have Millwall on the ropes and their counter-attacks have the home supporters nervous - but the Lions have shown plenty of defensive resilience during this spell of pressure, and if they manage to weather the storm, who knows what could happen?
24 minClaudio Ranieri looks composed on the touchline for a man under such pressure. His gameplan today appears to involve bombardment on the counter - a tactic which won them the league last term.
27 minCHANCE! More penalty box bombardment from Leicester as Ben Chilwell shoots from the edge of the area, only to see his deflected effort fall kindly for Steve Morison, who's back defending. He scrambles the ball clear and the Lions win their first corner of the afternoon at the other end...
29 minThe corner goes deep towards Cooper, who rises high at the far post and attempts to nod the ball across the face of the Leicester goal, but the ball ricochets awkwardly off his head and goes out for a goalkick.
32 minThe Foxes go route one as Yohan Benalouane plays a long ball forward to Demarai Gray. Jordan Archer races off his line and beats the Leicester forward to the lofted pass. Great vision from Benalouane, but I think Gray hesitated because he feared he was offside.
35 minArcher plays a goalkick long but Leicester gain possession and Gray goes on a mazy run into the Millwall penalty area, beating two defenders but running the ball out for a goalkick before he can square a pass into the six-yard box. Unlucky!
38 minMillwall look to apply some pressure of their own as Cummings's searching ball into the box almost finds Onyedinma in front of goal. Amartey does well to get their first and thump the ball clear with his head.
40 minLeicester have another sniff at goal as Musa twists and turns in the danger zone before shooting over from 25 yards. Millwall win a corner at the other end when Wague heads the ball behind under pressure from Morison but Zieler is quick off his line to claim it.
44 minIt's Leicester who are on the back foot now. Millwall have soaked up the pressure from their counter-attacks and have just won a corner after Onyedinma's dangerous cutback was put behind by Benalouane. Zieler claims the resulting cross unconvincingly, inviting a challenge from Cooper, who goes in on the keeper overzealously and earns a booking for his efforts.
3.49pmIt's goalless at The Den as we reach the halfway point of this cup contest, but we have a hard-fought and entertaining tie on our hands. Leicester were on top for a lengthy period before Millwall starting asking questions of their opponents.
3.50pmGray was a constant thorn in the Lions' paws during the first period and he's threatened to capitalise on the Foxes' counter more than once. However, Millwall soaked up that early pressure and had the visitors on the backfoot before the referee brought the half to a close.
3.52pmHaving backed Millwall before kickoff, I feel Leicester are now the favourites to progress with the scores tied at 0-0. They have strong reinforcements sitting on their bench - Vardy, Albrighton or Mahrez are capable of making an impact in a tie like this.
3.55pmLooking at the Millwall bench you have to ask whether Neil Harris is willing to risk throwing on Lee Gregory. He could certainly make an impact, providing his fitness holds up.
46 minKICKOFF! We're back underway at The Den. No changes for either side at the interval.
48 minHere's an action shot of Kaputska getting the better of Williams - but there's no telling which of these two will have the last laugh today with the scores level.
52 minRED CARD! Disaster strikes for Millwall as Jake Cooper picks up a red card for what can only be described as a reckless challenge on Ahmed Musa. He had already been booked so the referee had no choice but to give him his marching orders. That could prove decisive!
54 minBOOKING! Ron-Robert Zieler is controversially shown the yellow card as he collects the ball on the edge of his area, only to be barged outside of it by Fred Onyedinma. The referee rules in Millwall's favour, books the German and awards the hosts a free kick.
56 minThe Foxes scramble the resuling free kick clear and Musa leads a pacey break. A cross finds its way into King's path and he's inches away from connecting with a header that would surely have made the net ripple.
59 minThe early evidence suggests Millwall are coping well with their numerical disadvantage. They're still throwing just as many shirts back into the defence and inviting Leicester to press forward, no doubt hoping to hit them on the break in turn.
63 minThe Lions are still showing ambitions in the final third despite being a man down. The long ball picks out Fred Onyedinma on the edge of the Leicester box and he nods it down for Steve Morison to hit, but his strike flies high and wide.
64 minSUBSTITUTION: Leicester make a change and it's Bartosz Kapustka making way for Marc Albrighton. That will give the Foxes more width and add some pace to their counter-attack.
65 minMillwall are proving tough to break down and have actually been the better side since that sending off. Neil Harris's men have shown true grit against the Premier League champions.
67 minThere's a break in play as Molla Wague is down injured following a heavy 50-50 challenge. That will be the end of his afternoon - veteran Pole Marcin Wasilewski comes on in his place.
70 minLosing Wague could well have impacted on Ranieri's gameplan for the closing stages of this FA Cup tie. Jamie Vardy was warming up on the touchline before the enforced change was necessary.
73 minSUBSTITUTION: On comes Vardy regardless of the fact Ranieri has already made two substitutions. He replaces Gray in a like-for-like swap up front.
75 minSUBSTITUTION: It's Millwall's turn to switch things around and they've thrown on Nadjim Abdou from Callum Butcher. This was no doubt to add a fresh pair of legs to the equation - Butcher only recently returned from injury and was looking tired out there.
76 minFollowing Butcher's exit, here's an action shot of him going in hard on King. He's been solid in midfield this afternoon, operating in front of a tight defensive line.
79 minSUBSTITUTION: Another throw of the dice from Harris, who has thrown on Gregory for O'Brien despite the striker struggling to overcome a calf strain. Both managers are going for it with 10 minutes remaining. Will it be a grandstand finish?
80 minMISS! It's a glaring miss from Shinji Okazaki, who blows the opportunity to put the Foxes ahead from point-blank range. Albrighton's cross picks out the Japan international in front of goal, but he chooses to take a touch instead of smashing it first time. His hesitation allows Archer to pull off a last-ditch block!
83 minOkazaki's miss pretty much sums up Leicester's fortunes of late. Their confidence is in the earth's crust at the moment as they simply cannot score. Drawing 0-0 with a third-tier side who are down to 10 men will do little to rectify that.
86 minSAVE! Okazaki collects the ball inside the penalty area and attempts to play it towards Vardy in the six-yard box, but his cross is deflected goalwards and Archer is forced to react instinctively to palm it clear.
90 minGOAL! Millwall 1-0 Leicester City (Shaun Cummings)
90 minAbsolutely unbelievable! As if we haven't already had our fill of FA Cup shocks today, 10-man Millwall have stunned Premier League champions Leicester at the death. Right-back Shaun Cummings has delivered what could be a knockout blow, guiding the ball home after collecting Gregory's through-pass in the penalty area. Tremendous stuff at The Den!
90+5 minThere will be five anxious minutes of stoppage time for 10-man Millwall to withstand. A place in the quarter-finals of England's number one cup competition is at stake!
4.56pmThey've done it! Millwall have reached the quarter-finals of the FA Cup after stunning the Premier League champions at the death. This is the stuff cup dreams are made of - down to 10 men and a defender pops up in the final minute to fire your team through against all odds!
4.58pmYou have to say the Lions deserve it. Leicester didn't do enough to win the game and Millwall adapted to losing a man well. They continued to press forward undeterred and got their just desserts in the end.
4.59pmThere's a pitch invasion at The Den as the Millwall faithful celebrate defeating a Premier League side for the third time in a row. Their into the hat for the quarter-finals and the sky really is the limit given the rate that top-flight teams are dropping out of this competition!
5.00pmThat's all we have time for this evening but be sure to stick with Sports Mole for our ongoing coverage of this weekend's FA Cup action. Many thanks for reading.