Japhet Tanganga's goal was his first since signing a permanent deal at Millwall last summer
Japhet Tanganga's first goal of the season earned Millwall a fourth straight win as they held off Leeds and soared into the play-off places.
He volleyed in Jake Cooper's knockdown just before half-time at The Den and the defensive duo battled manfully throughout to protect that advantage.
Although Leeds piled on sustained pressure and Joel Piroe had a goal ruled out for offside, they could not find a response.
Millwall clung on to condemn the visitors to a first defeat in nine games, denying them the opportunity to regain a top-two place.
Lions boss Neil Harris made three changes from the side that had triumphed over Burnley on Sunday, with Danny McNamara, Ryan Wintle and Duncan Watmore all starting while Jayden Bogle returned to the Leeds line-up after suspension.
It was the visitors who dominated possession from the first whistle, with Wilfried Gnonto providing glimpses of attacking threat on the left flank, but they were unable to convert that into more than a brief cluster of chances.
Those came in quick succession as attempts from Brenden Aaronson and Piroe thudded innocuously through to Lions goalkeeper Lukas Jensen before Tanganga's block diverted a Gnonto drive behind.
Millwall's main threat came from set-pieces and they utilised a free-kick to snatch the lead, with George Saville picking out Cooper, who headed across the box for Tanganga to steer a volley beyond the diving Illan Meslier.
Leeds planted their flag firmly in the opposition half at the start of the second period and Gnonto's near-post header flashed over before Dan James cut inside to test Jensen from distance.
It looked as though the Yorkshire side had drawn level on the hour mark as Bogle's cross was touched on by James and Piroe swept into the net – only for the celebrations to be curtailed by a linesman's flag.
Leeds made a string of attacking substitutions and continued to pour forward, but Jensen was rarely troubled again as Bogle and James thumped attempts over the bar and Cooper charged down another Piroe drive.
One of the new arrivals, Mateo Joseph, almost gifted the home side a second goal five minutes from time, heading against his own post after Cooper had nodded another free-kick into the danger zone.
Meslier went forward for a Leeds corner deep into added time, but the Lions saw the game out for a fourth consecutive 1-0 success.
Millwall manager Neil Harris told BBC London 94.9:
"Let's be honest – Leeds were better than us the way they moved the ball, with the one-on-one ability and rotation they've got. For me they're going to win the league this year if they keep playing like that.
"We knew exactly what the game was going to be like, we knew the intensity was going to be high but the lads stood strong.
"For us it's about discipline, being organised and structured as we can be and then finding a way to score at the other end. I'm very proud of the players this evening.
"The two centre-halves were magnificent. Japhet got the goal, Coops got the assist but it's their defensive display that's so important to us – the amount of blocks they made and the way they defended crosses."
Leeds manager Daniel Farke told BBC Radio Leeds:
"I'm not annoyed at all with our performance – I thought we were excellent. This is a difficult place to go and I think we've created more situations in and around their box than the last four away teams here in total.
"You have to stay disciplined in each small situation. We were more or less playing in their half, but there will always be one or two set-piece situations.
"Football can sometimes be the most unfair sport in the world. If it's tennis, basketball or handball and you're dominating, the better side wins – but in football one goal is often decisive.
"It's also about efficiency to use your chances or to make sure there are not little mistakes and in these terms they were better than us today. We have to accept it and congratulations to Neil and his team."
The Leeds United manager was left frustrated as his side were beaten at Millwall.
