AN 85TH-MINUTE strike from veteran striker Steve Morison shattered Bradford City's Championship dream as their exile from the second tier extended into another season.
The former Leeds United frontman fired him clinically at the far post to settle the issue in a game in which the Bantams paid the price for missed chances.
Billy Clarke spurned a golden opportunity in the first quarter of an hour and Rory McArdle missed another great chance in the second period, while Tony McMahon fired into the side-netting at the death as City left their shooting boots at home.
Urged by Stuart McCall to display plenty of 'fire in their bellies and ice in their veins' City showed plenty of the former, but not enough of the latter, with the Londoners taking their chance when it came along late on.
McCall said: "It is fine lines in football. We had 60 per cent of possession but they put us under pressure from corners.
"It was a bit of brilliance from the goalkeeper (for save from Billy Clarke in first half).
"It just wasn't our day, unfortunately. Second half, we were a little bit quieter but still had opportunities."
City named an unchanged line-up from the side who took the field at Fleetwood in their semi-final second leg, while Millwall also started with the team who disposed of Scunthorpe United in their own pivotal second instalment.
The two games between both sides in the regular season may have proved nip-and-tuck by way of two 1-1 draws, but an open spectacle ensued from the off, with Bradford, utilising a fluid 3-5-2 formation with Romain Vincelot in the heart of defence, being the most dominant force in the first period after an even opening, but failing to land a killer blow.
An early strike from McMahon drifted wide, with the Lions also displaying danger in the opening sparrings, with Nathaniel Knight-Percival doing well to prevent Steve Morison from getting a strike on goal.
A mini-flurry of well-struck corners from Shaun Williams soon tested the Bantams' famed defensive mettle, with Tony McMahon ensuring Shaun Hutchinson could not get a clean header from one such flag-kick before a full-stretch Morison headed off target following another Williams corner.
Play soon switched to the other end and so nearly yielded a Bradford opener on 14 minutes.
A counter-attack at express pace left Millwall exposed with a perfectly weighted pass from Mark Marshall sending Billy Clarke clear, only for Jordan Archer to race out and get his angles right and make a fantastic finger-tip save to keep out his goalbound effort.
It was a huge moment in the context of the game and soon after, City pressed again with James Meredith afforded space on the left before unleashing a fierce cross-shot , which was beaten away by Archer.
City - - roared on by 24,000 supporters clad in claret and amber - displayed clear threat on the counter-attack and a promising situation was spurned on 24 minutes with Hutchinson getting in the way of McMahon's goalbound free-kick after Jimmy Abdou felled Clarke.
Just after the half-hour mark, Archer grasped Rory McArdle's header after the defender got up highest following McMahon's deep corner, with Bradford, whose approach play was eye-catching, starting to boss proceedings, while utilising the wide open spaces of Wembley to the full.
The final moment of note in a half which Bradford increasingly dominated arrived when Charlie Wyke's glancing header flew wide following McMahon's free-kick.
The flowing football in the first period arrived from Bradford, with Millwall displaying threat from set-pieces, as opposed to in open play in a clear contrast in playing styles.
The start of the second half followed much of the pattern of the first with City posing the problems with Meredith seeing an early shot blocked as the Bantams pressed.
A golden chance fell Bradford's way on 58 minutes, but once again, it was a moment which the Yorkshire outfit were forced to rue.
City were awarded a free-kick in a dangerous position after Aiden O'Brien picked up a caution for bringing down Marshall and from McMahon's inviting set-piece, an unmarked McArdle had a clear sight of goal, but couldn't keep his header down, with his point-blank header flying over.
It was then the turn of Millwall to lament their profligacy just after the hour mark, with Jed Wallace missing a glorious chance to fire the Londoners in front after being sent clear by Lee Gregory's slick pass, with the midfielder's effort flashing wide of Colin Doyle's right-hand post.
It was anyone's game with McArdle soon nodding over again following another McMahon free-kick in a game where one goal was increasingly looking like it would prove enough for either side.
Proceedings were on a knife edge with Meredith soon booked for clattering into Wallace before a last-ditch challenge by McArdle denied Morison and bailed out Vincelot in the process after his initial poor clearance.
At the other end, Meredith guided a floated header wide of Archer's goal following McMahon's centre as play swung to the other end in a half which were certainly more even than the first period with Millwall imposing themselves on the game much more.
With extra-time looking an increasing possibility, Millwall broke City's hearts with the game-breaking moment five minutes from time.
Williams' left-wing centre was flicked on by Gregory and the lurking Morison clinically dispatched the ball high past Doyle on the volley to break the deadlock.
The adding on of five minutes of stoppage time gave City an inkling of hope, with McMahon firing into the side-netting in a dramatic finale, but the Londoners held out to book their Championship berth with City's crest-fallen players fell to the deck at the final whistle.
Bradford City: Doyle; McArdle, Vincelot, Knight-Percival, McMahon, Law (Dieng 74), Cullen, Meredith; Clarke (Jones 74); Wyke, Marshall. Substitutes unused: Sattelmaier, Darby, Hiwula, Gilliead, Kilgallon.
Millwall: Archer; Romeo, Webster, Hutchinson, Craig; Wallace (Onyedinma 85), Williams, Abdou, O'Brien (Ferguson 69); Morrison, Gregory (Butcher 89). Substitutes unused: King, Cooper, Thompson, Worrall.
Referee: S Hooper (Wiltshire).