David Somma stepped off the bench to bag a brace as Leeds United finally toppled The Lions to condemn Millwall to their first defeat of the season.
Having taken a 15th-minute lead courtesy of a headed own goal from former Lion Richard Naylor, Lloyd Sam levelled on 32 minutes before sub Somma struck twice in the final 11 minutes to secure United's first Championship win of the season.
Following six epic encounters between the two teams during the past couple of seasons during which The Lions had been beaten just once, Kenny Jackett's men went into Saturday's clash at Elland Road top of the Championship table in a quietly confident mood.
But it was Simon Grayson's Yorkshiremen who ran out deserved winners, having really put Millwall under the cosh for long spells of the game.
The opening half was played a breathtaking pace and in the early stages, the two teams exchanged scoring opportunities as play swung from one end to the other.
Sanchez Watt was a constant thorn in the side of the visiting defence and he had already seen an audacious 25-yard chip drop onto the roof of the net before picking out Luciano Becchio, who dragged wide from a central position.
It took a booming clearance by Leeds keeper Kasper Schmeichel to avert the danger when Kevin Lisbie chased forward to connect with Danny Schofield's dinked pass and then James Henry's crisply struck low drive was saved by the keeper at full stretch.
Millwall broke the deadlock on 15 minutes and although former Lion Naylor provided the final touch - a bullet header into his own net - the goal owed as much to the superb persistence and vision of Morison, who won possession on the right flank, held off Federico Bessone and somehow squeezed over a cross that Naylor converted under pressure from Lisbie.
Thereafter it really was backs to the wall for The Lions, who were put under the most intense spell of attacking pressure.
Becchio was guilty of a horror miss when he contrived to blast high and wide with the goal at his mercy from no more than six yards.
Some fancy footwork by Johnny Howson saw the Leeds midfielder skip into the area before smacking the foot of the post.
Keeper David Forde then came to the rescue with an important one-handed punch to tip over Sam's lob before the home side finally grabbed the equaliser their pressure warranted.
Watt slipped the ball through to Becchio, he touched into the path of Sam and the former Charlton winger swept past the outstretched hands of Forde.
Grayson's men continued to pile forward with Darren Ward clearing off the line from Sam and the woodwork came to the rescue once more; this time to keep out Neill Collins' header from a Neil Kilkenny corner.
At the other end Jackett's men fashioned a rare scoring chance when Henry's low free-kick was well held by Schmeichel.
The second-half didn't match the same intensity as the first 45 minutes to begin with, as The Lions looked to try and get a foothold in midfield to contain a lively Leeds side.
It was substitute Darren Carter, a 55th-minute replacement for Lisbie, who went close to handing the visitors an unlikely lead four minutes after his introduction, only to glance a header from Henry's cross narrowly over the bar.
Carter was involved in the thick of the action again when he scooped an effort just off target from 18 yards as The Lions tried to build up a head of steam.
But it was Leeds who gradually began to dominate proceedings once more and Forde did well to gather at the second attempt from a stinging Howson effort as Grayson's men took control.
Despite having so much of the possession, Leeds had looked to be in danger of paying for their profligacy in front of goal.
But the introduction of striker Sommer with 16 minutes remaining proved decisive.
Within just three minutes the irrepressible Watt returned a cross from the right into the danger zone and Somma hammered home to put Leeds 2-1 ahead.
Just seconds later Liam Trotter swivelled and hit a fierce first-time shot from Carter's cross that looked destined for the roof of the net until Schmeichel somehow got his fingers to the ball to push over.
But Millwall could have found themselves 3-1 behind as play swung up the other end, this time Sam's header from another inviting Watt delivery hitting the post.
With The Lions pushing forward in a desperate search for an injury-time equaliser there was always a danger of being caught out on the break - and that's just what happened in the 94th-minute as Somma sent the Elland Road faithful into ecstasy with a rasping, rising angled shot to wrap up three deserved points.
Leeds United: Schmeichel, Connolly, Collins, Naylor, Kilkenny, Becchio, Sam (Hughes, 90+1mins), Howson, Johnson, Watt, Bessone (Somma, 74mins). Subs not used: Higgs, Bruce, Grella, Clayton, White.
Scorers: Sam (32mins), Somma (79, 90+4mins)
Millwall: Forde, Dunne, Barron, Abdou, Robinson, Ward, Schofield, Trotter, Morison, Lisbie (Carter, 55mins), Henry. Subs not used: Mildenhall, Harris, Hackett, Mkandawire, Smith, Marquis.
Scorer: Naylor (og 15mins)
Ref: Colin Webster