Super sub Gary Alexander stepped off the bench to send Leeds spinning to their first League defeat of the season in a pulsating encounter at The Den.
Alexander, a 60th-minute replacement for Steve Morison, produced a thumping header to power Danny Schofield's cross past the outstretched hands of United keeper Casper Ankergren.
Neil Harris had handed The Lions a perfect start after just three minutes, heading Chris Hackett's free-kick home, only for Paddy Kisnorbo to haul the visitors level eight minutes later.
The game looked to be heading for a draw, but then up popped Alexander to bag his first League goal since April to send the home fans wild - and put Kenny Jackett's side within one point of the Play-Offs.
Make no mistake, this was a thoroughly deserved victory by a gutsy Lions side who, to a man, worked tirelessly to grind out this success.
From the outstanding Tony Craig, unbeatable in the air and on the deck throughout, through the tireless Marc Laird in midfield and the talismanic Harris up front, this was a performance to savour.
Lions boss Kenny Jackett had the luxury of naming an unchanged side for the second successive game, and it took just three minutes for The Lions to break the deadlock.
Morison had already seen his effort in the opening minute blocked for a corner as Jackett's men came flying out of the traps.
When Harris was sent sprawling by Richard Naylor on three minutes, Hackett clipped a deep delivery into the box and picked out Harris, who in turn slipped his marker and sent a pinpoint header into the roof of the net.
In a frantic opening spell, Leeds hauled themselves back onto level terms after 11 minutes courtesy of Kisnorbo's fierce close-range drive when The Lions had failed to clear Bradley Johnson's corner.
Back came Millwall with real purpose and passion.
Great work by the tricky Hackett and Harris on the right saw the latter's deft delivery curl inches away from Morison and Laird, whilst another accurate pass from Hackett picked out Morison, who in turn saw his cross deflected for a corner.
Craig was left cursing his luck on 32 minutes after sending his header from another Hackett cross a yard wide with that man Hackett himself sending a low drive narrowly off-target following great build-up play between Alan Dunne and Smith.
After the break Millwall took control of the game for long periods but were unable to convert their chances.
Hackett in particular put in put in a series of teasing crosses, and from one Craig should have put The Lions back in front but saw his header flash narrowly wide.
Morison too was left holding his head after finishing an excellent move down the left by nodding the cross straight into the grateful arms of Ankergren.
But with Laird and Jimmy Abdou tigerishly winning possesion in midfield, full backs Jack Smith and Alan Dunne supporting the attack at every opportunity, and Schofield balancing Hackett's endeavours on the left, there was always the threat that The Lions might score again.
Morison got a standing ovation from the fans for his efforts when he was replaced by Alexander on the hour, and it proved to be a master stroke from boss Jackett.
There were eight minutes left on the clock when Smith did superbly to keep the ball in play and set Schofield free down the left. The number seven got to the byeline and pulled back a perfect cross onto Alexander's head for what proved to be the winner.
Leeds, who had barely threatened to that point then moved up a gear.
Sub Tresor Kandol crashed a 20 yarder against a post and Paul Robinson cleared off the line in a frantic finale, but anything other than a Millwall win would have been an injustice on the balance of the second half play.
Millwall: Forde, Dunne, Smith, Abdou, Robinson, Craig, Schofield, Laird, Harris (Price, 88mins), Morison (Alexander, 60mins), Hackett. Subs not used: Sullivan, Henry, Martin, Bolder, Grabban.
Bookings: Robinson (56mins), Alexander (90+4mins)
Scorers: Harris (3mins), Alexander (84mins)
Leeds United: Ankergren, Kisnorbo, Doyle (Kilkenny, 88mins), Naylor, Beckford, Howson, Vokes (Kandol, 70mins), Johnson, Hughes, Snodgrass (Gradel, 78mins), Bromby. Subs not used: Fielding, Crowe, Prutton, Michalik.
Scorer: Kisnornbo (11mins)
Ref: Mark Clattenburg
Att: 14,165 (away 1,993)