JED Wallace struck the only goal of the game to give Millwall just a second Championship win away from home this season and move his side five points clear of the relegation zone.
Wallace ran onto a pass from Shane Ferguson in the 72nd minute to sprint clear and beat Kelle Roos with a low left-footed drive.
Derby had their first effort on target in injury-time, but Jordan Archer sprung low to his left to save Fikayo Tomori’s header.
Millwall could not have sprung their trap better on Derby. The hosts had most of the possession but did little with it. Still, if a goal was going to come you would have thought it more likely it would be the home side breaking through.
But the Lions have been in this position so often under Neil Harris, defending in numbers, seeing the opposition commit an extra player or two forward, and then striking on the break.
If Wallace wants a regular place as a forward this was a good way to make his case. Like his goal at Middlesbrough he used his pace and it took him clear of Ashley Cole. He had a similar chance late on at AFC Wimbledon on Saturday but didn’t shoot and was tackled. Maybe that memory came back to him and this time he was determined not to be caught. His shot was the first on target in the game. Lee Gregory's role in it was also crucial. He often had a thankless task, but always showed for the ball. It was he who collected a clearance from Ryan Leonard from a Rams free-kick to allow an attack for the winner.
Derby had a late chance through Tomori, but Archer was equal to it, as Millwall handed Frank Lampard's side just a second home league defeat since August.
Millwall boss Harris made two changes to the side that defeated AFC Wimbledon at the weekend. Shaun Hutchinson returned in place of Alex Pearce, who couldn’t play against his parent club under the terms of his loan deal.
Ben Marshall started in place of Aiden O’Brien.
4-4-2: Archer; Romeo, Hutchinson, Cooper, M Wallace; Ferguson, Leonard, Williams, Marshall; J Wallace (Tunnicliffe, 89); Gregory (Elliott, 75).
Derby dominated possession in the first half against a Millwall side with Wallace in support of Gregory up front.
The Rams pushed their wing-backs Ashley Cole and Duane Holmes high up the pitch to try to stretch the visitors, with Tom Huddlestone sweeping passes out to the flanks. But his radar was wonky as often as not. Cole and Huddlestone were substituted in the second half.
Millwall defended with discipline, frustrating the home fans. The hosts had one glimpse of goal in the opening half an hour, but Martyn Waghorn over-ran the ball in the box after being picked out by Holmes.
Derby were successful in not giving Millwall chances to cross, Mahlon Romeo and Ferguson joined by Marshall from the left wing but frustratingly not being able to pick a way through. Cole’s class was evident in that area and he was well backed up by Tomori.
Gregory was seeing as little of the ball as Jack Marriott, who barely had a significant touch in the first half.
In the 32nd minute Derby got an opening when Hutchinson blocked Huddlestone’s shot. The ball spun to Waghorn but he couldn’t twist an effort on target. Five minutes before the break Huddlestone, with little movement in front of him, shot from 30 yards but Archer watched it sail over.
About the most interesting part of the opening period was Derby attempting to play through the Lions’ high press. They did it reasonably well but lacked imagination further up the pitch.
Millwall’s best opportunities looked like they would come from set-pieces, but Rams goalkeeper Roos was not being tested.
Lampard’s side claimed a penalty just before half-time when Marriott appeared to be held back, but referee James Linington didn’t agree.
The second half began in the same pattern as Derby moved possession wide. They got an early corner that centre-back Richard Keogh flicked wide at the near post.
Moments later Marriott produced a lovely touch to get between Murray Wallace and Jake Cooper and it looked like he was in, but fell over as he shot.
Ten minutes into the second half Wallace’s cross from the left appeared to clip the bar as Roos was finally given something to concern him.
A decent spell of Millwall possession ended with Romeo’s cross to Wallace, but under pressure the attacker swiped at the ball.
Wallace got another chance with 18 minutes left, and this time it was a very different outcome. Ferguson was quick to spot the pass, and Wallace beat Cole before shooting on his left foot. A little corner of Pride Park exploded in noise as the Lions fans present let Derby know their team doesn’t often win away from home.
The Rams couldn’t respond, as the Lions did the league double over them for the first time since 2004-05.
Image: Millwall FC