A nightmare first-half saw Leeds trail to goals from Aiden O’Brien and Lee Gregory, with an awful half compounded by a third Whites dismissal in three matches, with captain Liam Cooper receiving his marching orders for a high challenge on George Saville.
But Leeds, fuelled by a raucous crowd, staged a stunning rally to turn the tables and lead 3-2 thanks to a double from the recalled Pierre-Michel Lasogga and Kemar Roofe, only for the Lions to add the late sting through goals from ex-Leeds man Tom Elliott and Wallace.
It proved a defensively choatic first half of the highest order, with the tone set inside the first twenty seconds, with Steve Morison setting up the untracked O’Brien, whose effort was turned away by Felix Wiedwald.
Leeds did show hunger and intent, briefly, at the other end, only for a collective dereliction of defensive duty and discipline to provide a welter of encouragement for the visitors, who seized on the hosts’ palpable failings.
It yielded two goals for the Londoners and it could - and probably should - have been more, although the damage was exacerbated by the red mist descending once again.
It was a half in which United old boy Morison, on his first return to Elland Road, had an absolute ball, setting up both goals and having his moment in front of the Kop in the process.
After an early warning, Leeds did have plenty of encouragement in front of the South Stand, but unfortunately, the recalled Pierre-Michel Lasogga saw his goalscoring radar go awry.
First, he saw his point-blank header fly straight at Jordan Archer following Pablo Hernandez’s centre before he spurned another golden chance, with Archer racing out to block his effort after good work by Kalvin Phillips, who dispossed Jed Wallace.
The important of those dual misses soon became evident.
Millwall, with Morison in dominant form, started to carve open Leeds’ flaky back four and he soon played in Gregory, who should have scored after racing clear, only to take an extra touch and drag a shot wide.
Soon after, the Lions had the ball in the net, with a low shot from the edge of the box from Wallace flying into the net, only for the officials to spot a marginal offside offence.
But the respite was somewhat brief, with Morison getting away down the right before delivering an inviting cross, which was headed in by O’Brien, who got about Gaetano Berardi - with both he and Morison milking the occasion.
An atrocious defensive showing of staggering ineptitude then saw O’Brien left unmarked into the box following Wallace’s free-kick, only to nod over before Jake Cooper soon headed into the Kop.
The pressure continued with Shaun Hutchinson again winning the air miles to see his header clip the bar before a fraught half turned into scarcely believable one eight before the break when Liam Cooper saw red after his challenge on Saville, with the visiting bench suitably incensed.
After a commotion between both benches, the United captain recieved his marching orders, but a vat load of salt was rubbed in the hosts’ wounds.
It came when Morison outmuscled Vieira and got away on the right once more with his low shot turned in from close in by Gregory and there was still time before the break for Wiedwald to deny Wallace.
It had the makings of a long second half for Leeds, but another twist in a dramatic game arrived at the start of the second half.
Shaun Williams erred and Kemar Roofe seized upon his indecision to set up Lasogga, who started the atonement process following his earlier misses by emphatically firing the ball and provide home supporters with a big adrenaline rush.
An increasingly pulsating affair then saw parity restored as Millwall creaked amid a cauldron of raucous white noise and more especially Archer.
The Millwall keeper fluffed a routine centre from debutant Laurens De Bock and amid the melee in a crowded box, Roofe bundled in a sweet leveller.
The visitors were wilting visibly, with it increasingly unfathomable that it was they who had an extra man and not the hosts, feeding off a pumped-up crowd.
United’s remarkable turnaround was then sealed when Hernandez teed up Lasogga, who showed a classy touch to fire an inch-perfect low drive past the clutches of Archer.
Millwall vainly attempted to regroup with substitute and former Leeds man Tom Elliott heading wide before a timely clearance from another replacement in Stuart Dallas denied Gregory close to the line as Leeds again failed to convince at the back.
Unfortunately, Leeds were exposed before the end with Elliott nodding in and Wallace’s late, late winner proved the final act as the visitors sealed a double over Leeds, whose winless streak stretched to five games.
Leeds United: Wiedwald; Berardi, Jansson, Cooper, De Bock; Hernandez, Vieira (Pennington 43), Phillips, Alioski; Lasogga (Shaughnessy 83), Roofe (Dallas 74). Substitutes unused: Lonergan, Anita, Sacko, Cibicki.
Millwall: Archer; McLaughlin (Romeo 82), Hutchinson, Cooper, Meredith (Elliott 66); Wallace, Saville, Williams, O’Brien (Ferguson 63); Morison, Gregory. Substitutes unused: Martin, Craig, Tunnicliffe, Thompson.
Referee: David Coote (West Yorkshire).