First-half goals from Steve Morison and Jason Puncheon secured victory as 10-man Millwall signed off a memorable 2010 on a winning note at The Den.
The 54th-minute dismissal of stand-in skipper Alan Dunne for an impetuous two-footed challenge on Richie Wellens saw The Lions play the last 36 minutes at a numerical disadvantage.
Whilst Millwall's attacking play reaped what proved to be a crucial two-goal lead in the first half, it was an outstanding defensive performance during the second period that stifled any hopes Leicester had of clawing their way back into this match.
Millwall may have been a man down in numbers, but the immense input of the home crowd should not be underestimated. When the players needed a lift, the noise reverberating around The Den did just that and helped carry the team over the finishing line.
"It was a big performance, we raised our game, both the supporters and the players," said Kenny Jackett.
"Leicester are a big club, so to overcome them is a good result. We were clinical in taking our chances, saw it through and I felt it was the right result in the end."
On the second-half dismissal of Dunne, Kenny added: "I have got no complaints with the decision. If you take your feet off the ground in this day and age you will be in trouble."
Millwall dominated a disjointed opening first 45 minutes, but it was Leicester who had the first effort on goal after seven minutes when Paul Gallagher lashed the ball wide of the mark after beating the offside trap.
Four minutes later Millwall made the breakthrough when Liam Trotter, again playing further upfield in a 4-4-1-1 formation, produced a powerful run into the danger zone. Trotter spotted Morison in space 12 yards out, and he produced a fine finish, swivelling and hooking home with his right foot for goal number 11 of the season.
The Lions doubled their tally 12 minutes later courtesy of Puncheon's second goal in a Millwall shirt. The loanee from Southampton had an effort half-cleared by the Foxes defence before Darren Ward's backward header played Puncheon in, and he made no mistake at the second attempt with a clinical low finish that gave keeper Chris Kirkland no chance.
James Henry went close with a curling 25-yard free-kick a couple of minutes later as The Lions went in search of a third goal.
Although Leicester huffed and puffed, the visitors came up against a compact Lions rearguard that was well protected, with midfield duo Tam Mkandawire and Jimmy Abdou at their best.
It took Leicester until the 41st minute to conjure up only their second attempt on goal, a fizzer from Kyle Naughton that flew wide of the left hand post.
Then on the stroke of half-time Lloyd Dyer and Naughton combined on the right, with the latter's hopeful delivery scooped by Gallagher into the upper tier of the South Stand.
The Foxes made a double substitution at the start of the second half with keeper Kirkland replaced by Chris Weale, and ineffective frontman Roman Bednar swapped with former England international Darius Vassell.
With Gallagher pushed up front to partner Vassell, The Foxes flew out of the traps and within seconds of the restart it took a brlliant save by Forde to divert Dyer's goalbound effort away for a corner, although his touch was missed by the officials who awarded a goal-kick to Millwall.
Millwall were forced to regroup on 54 minutes when Dunne's rash challenge on Wellens left referee Mark Russell no option than to brandish a straight red card.
Mkandawire switched from midfield to fill the right-back berth, keeper Forde took over the captain's armband and Danny Schofield entered the fray replacing Henry shortly after as The Lions reshuffled their pack.
In fact Schofield nearly marked his introduction in sensational style, his first touch nipping in to take advantage of hesitancy in the visiting rearguard, slipping the ball to Morison on the right with the latter's delivery a whisker away from the lurking figure of Puncheon eight yards out.
Leicester boss Sven Goran Eriksson introduced striker Steve Howard, hoping that the physical presence of his big front man would put the Millwall back four under more pressure.
But although Leicester had plenty of possession, their attacking play lacked an incisive final pass and with the home crowd in full, raucous voice, Jackett's men stood firm.
The visitors only clear attempt on goal in the final 20 minutes was not even of their own making; the otherwise impeccable Ward dithered in the penalty area, Howard was quick to react but contrived to send his angled drive wide of a gaping goal.
Tony Craig felt the full force of Matt Oakley's long-range injury-time piledriver, making a brilliant block to ensure The Lions signed off December in style.
Millwall: Forde, Dunne, Barron, Abdou, Ward, Craig, Puncheon (Hackett, 89mins), Mkandawire, Trotter, Morison, Henry (Schofield, 60mins). Subs not used: Mildenhall, Lisbie, Smith, Marquis, McQuoid.
Booking: Forde (86mins)
Sending off: Dunne (54mins)
Scorers: Morison (12mins), Puncheon (24mins)
Leicester City: Kirkland (Weale, 46mins), Morrison, Gallagher, Oakley, King, Dyer, Naughton, Wellens (Howard, 61mins), Hobbs, Cunningham, Bednar (Vassell, 46mins). Subs not used: Waghorn, Berner, Abe, Moussa.
Ref: M. Russell
Att: 12,188 (away 1,355)