Aiden O'Brien's 73rd-minute strike earned Millwall a more than deserved three points against Leeds United at The Den in the Sky Bet Championship on Saturday afternoon.
The home side overpowered the League leaders from start to finish and the Irishman's strike secured Millwall their second Championship win of the campaign.
Lions boss Neil Harris made two changes from Tuesday night's 2-2 draw at Queens Park Rangers - one enforced - with Fred Onyedinma and O'Brien replacing the suspended Lee Gregory and Shane Ferguson, who was named among the substitues.
A predictably frenetic opening saw The Lions have the ball in the net in on 7 minutes, but Steve Morison was ruled to be offside after turning home Conor McLaughlin's shot, which was spilled by Leeds goalkeeper Felix Wiedwald.
Jed Wallace caused the away defence problems with two delightfully whipped in crosses, but no Millwall head or foot was able to turn the ball home to open the scoring.
The winger had the next real attempt of the game on 33 minutes, but his shot was well over the crossbar. Two minutes later, Wallace powered past his marker and delivered another cross, but the ball was just taken off the feet of Morison - who would have had a tap-in - for a corner-kick.
Millwall were making the large majority of the attacking play and saw a well-worked free-kick routine deflected behind on 39 minutes, whilst Morison's header had Wiedwald scrambling to save two minutes before the break. Shaun Williams forced the stopper into a save seconds later as The Lions upped the ante before the whistle blew. Wallace had the final attempt of a half that the home side dominated from start to finish.
The second-half picked up where the first left off, with Wallace's deflected cross being palmed behind by Wiedwald. The 'keeper was called into action once more on 50 minutes as Shaun Hutchinson's goal-bound effort was well stopped.
Wallace was causing Leeds untold problems and nearly got his reward shortly before the hour-mark, but his strike crashed against the foot of the post.
The Millwall pressure was relentless as the final third of the afternoon approached - Morison almost had his first goal of the season, but his header was hooked off the line. On 71 minutes, the search continued, but once more Wiedwald was equal to Morison's drive.
Two minutes later, after a completely dominant display over the League leaders, The Lions more than deservedly took the lead through O'Brien. Morison laid the ball off to Wallace, who crossed into the penalty area where three unmarked Millwall players were waiting. The Irishman was the one tasked with finally beating Wiedwald with a low drive.
As a frantic final period set in, The Lions could have had a second after Wiedwald was caught wondering outside his penalty area, but indecision allowed Leeds to take the ball in search of an equaliser.
Thankfully, that didn't arrive and Millwall inflicted a first League defeat of the season on their rivals.
Millwall: Archer, McLaughlin, Meredith, Hutchinson, Williams, Onyedinma (Cooper 90+4'), Wallace (Tunnicliffe 90'), Webster, Morison, O'Brien (Ferguson 81'), Saville. Subs not used: Martin, Craig, Romeo, Twardek.
Scorer: O'Brien 73'
Leeds United: Wiedwald, Ayling, Anita, Lasogga, Alioski (Roofe 46'), O'Kane, Jansson, Hernandez (Dallas 46'), Saiz (Grot 71'), Phillips, Shaughnessy. Subs not used: Lonergan, Cibicki, Vieira, Berardi.
Booking: Shaughnessy, Saiz, O'Kane
Referee: Tim Robinson