Jake Cooper and Shaun Williams were on target as Millwall claimed their first Sky Bet Championship win of the season on Saturday afternoon as Derby County were defeated 2-1.
Cooper got the final touch to open the scoring on seven minutes, with Williams' deflected effort doubling the advantage later into the first-half.
David Nugent reduced the arrears on 73 minutes, but The Lions held firm to claim all three points at the expense of Frank Lampard's men.
Lions boss Neil Harris opted to revert to the starting XI and substitutes from last Saturday's 0-0 draw at Blackburn Rovers, with James Meredith once again the sole survivor from Tuesday night's penalty victory over Gillingham in the Carabao Cup.
With just a minute on the clock Cooper sent a searching header from Williams' corner-kick towards goal but the ball was cleared to safety. Meanwhile, at the other end, Jordan Archer had to be on form to tip Tom Lawrence's long-range free-kick over the crossbar.
Seven minutes in The Lions took the lead through Gregory - a low free-kick bobbled into the penalty area and the striker gained the deftest of touches to turn the ball home for his second goal of the campaign. It was perhaps one for the dubious goals panel, however, as the final touch seemed to come from Cooper.
As the atmosphere from three sides of The Den rose a notch, Meredith beat his marker and sent in a cross for George Saville, but the midfielder blazed wide.
Twenty minutes in, it was two. Gregory's shot was blocked but the ball fell into the path of Williams - the Irishman unleashed a shot from around 30 yards out that looked to be testing Scott Carson, but a deflection took it away from the stopper's grasp and into the top corner of the net.
The visitors attempted to threaten as the half-hour mark approached, but it was The Lions who had the next chance, with Saville shooting wide.
In the last action of a strong first-half for The Lions, Aiden O'Brien blazed over the bar from Williams' corner and Cooper's cushioned header that followed.
Referee Stephen Martin rightfully waved away penalty appeals from the Derby support as Martyn Waghorn tumbled under pressure from Cooper as the second-half got underway, whilst Craig Bryson sliced a shot over the bar.
The Rams had the ball in the net on 54 minutes through Waghorn, but the striker was correctly ruled offside by the assistant referee on the far side. O'Brien then showed skill and tried his luck eight minutes later but his effort went wide.
Mason Mount found himself in space but fired wide with 20 minutes remaining, whilst substitute David Nugent was perhaps lucky to escape an early bath following a late challenge on Shaun Hutchinson a minute later.
The attacker was in the thick of the action once more on 73 minutes as he found room to blast home Derby's opening goal of the game. Millwall could have responded with a third minutes after that but Saville rolled the ball agonisingly wide after latching on to Hutchinson's knock-down. Gregory then took advantage of indecision in Derby's back-line but failed to convert.
The Lions were now having to defend from the Derby onslaught and were limiting The Rams to efforts from distance - Bryson's shot wide from 25 yards out a prime example. Ryan Tunnicliffe's introduction saw The Lions switch to a five-man midfield and the former Manchester United man was nearly involved a third goal, but Saville shot wide.
After three minutes of injury-time referee Martin blew his whistle and The Den erupted - the unbeaten League run now stretches to four games.
Millwall: Archer, Meredith, Hutchinson, Cooper, Williams, J. Wallace (Ferguson 84'), Gregory (Tunnicliffe 84'), Romeo, Morison, O'Brien, Saville. Subs not used: Amos, McLaughlin, Elliott, M. Wallace, Skalak.
Scorer: Cooper 7', Williams 20'
Derby County: Carson, Bryson, Tomori, Keogh, Mount, Waghorn (Nugent 67'), Lawrence, Jozefzoon (Bennett 67'), Johnson, Bogle, Malone (Forsyth 46'). Subs not used: Roos, Widsom, Marriott, Evans.
Scorer: Nugent 73'
Referee: Stephen Martin
Attendance: 13,557 (1,436)