Derby County: Grant, Forsyth, Bryson, Keogh, Hendrick (Ward, 62mins), Sammon (Bennett, 80mins), Hughes, Buxton, Dawkins (Russell, 63mins), Wisdom, Bamford. Subs not used: Legzdins, Eustace, Whitbread, Thorne.
Steve Morison rose to head home his sixth goal of the season as Millwall turned the form book upside down to record a shock 1-0 win at high-flying Derby County.
Striker Morison - one of four changes to the side beaten by Brighton last weekend - marked his return in style as he produced a textbook 61st-minute header from Lee Martin's deep cross to secure only The Lions' second away victory of the season.
Whilst the stats may favour Derby in terms of possession overall, there could be no disputing that this was a performance of the utmost guts, composure and professionalism as The Lions played out seven minutes of injury-time a man down after Scott Malone hobbled off injured.
Boss Ian Holloway tinkered with his starting line-up in a bid to find that elusive win, bringing in right-back Alan Dunne, strikers Morison and DJ Campbell along with midfielder Lee Martin.
As expected, Derby came sprinting out of the traps, looking for an early goal on which to build what was, on paper, deemed to be an odds-on home win.
However Ollie's men had other ideas and in the opening 45 minutes defended stoically, whilst also carving out a couple of decent chances for themselves.
Craig Bryson's early free-kick swung over from the left, picked out Patrick Bamford, but the striker's flicked header drifted wide.
The Rams continued to ask questions of the visiting rearguard, but keeper David Forde was alert to thwart Conor Sammon after Craig Forsyth's cross into the box was knocked down by Bamford.
Forde then watched a Simon Dawkins effort sail over his crossbar, whilst at the other end Ed Upson drilled the ball straight into the grateful arms of Rams stopper Lee Grant with Millwall's first chance of the game.
Upson, a driving force in midfield alongside Shaun Williams, then hooked an effort wide and left-back Malone lashed over from inside the box as Millwall looked to shock the home side before the break.
The Lions lost Danny Shittu through injury 10 minutes after the restart, with Paul Robinson slotting into the back-line, but just past the hour mark The Lions got their noses in front - and what a crucial goal it could prove to be.
Martin did well to make time and space to pump over an inviting delivery from the left flank, keeper Grant got nothing on the ball and Morison took full advantage with a firm header into the back of the net from close range.
That prompted Rams boss Steve McClaren to make a double substitution as he brought on Jamie Ward and Johnny Russell for Jeff Hendrick and Dawkins.
But as The Rams threw numbers forward in a bid to claw their way back, it was Millwall who actually carved out the better chances.
Morison was only denied his - and The Lions' - second goal when he latched onto Jake Buxton's underhit back pass. The frontman did everything right, hitting his goal-bound shot with pace and power, but keeper Grant dived athletically and got his right hand to somehow clear the danger.
Grant was equal to efforts from Owen Garvan and then sub Martyn Woolford as The Lions could have added to their tally.
Even when left-back Malone limped off leaving the team down to 10 men during seven minutes of injury-time, The Rams could not find a way through an impenetrable Lions rearguard as Ollie's men held firm for a massive three points.
Millwall: Forde, Dunne, Malone (off inj, 90mins), Shittu (Robinson, 56mins), Lowry, Martin (Woolford, 65mins), Upson, Williams, Campbell (Chaplow, 75mins), Morison, Garvan. Subs not used: Bywater, Fredericks, Jackson, Abdou.
Bookings: Forde (90mins), Dunne (90mins)
Scorer: Morison (61mins)