Chris Wood bagged a hat-trick as The Lions suffered a bad case of the Blues in Sunday's St Andrews lunchtime showdown.
The teenage striker, signed on loan from West Bromwich Albion last month, hit a textbook treble with goals on 28, 62 and 90 minutes to lift Chris Hughton's Birmingham City outfit out of the drop zone.
Having seen Scott Dann and Cameron Jerome leave St Andrews on transfer deadline day with moves back to the Premiership, and also endured a stuttering start to the campaign following relegation from the top flight in May, there was a somewhat subdued atmosphere amongst the home fans prior to kick-off.
And it was Kenny Jackett's side, featuring debutants Liam Feeney and Jay Simpson, who set the tempo from the off to heighten the anxiety.
Birmingham had to withstand a lot of early Lions pressure with Hamer Bouazza twice going close in the opening seven minutes.
The winger's first attempt came on three minutes when Curtis Davies was penalised for holding Darius Henderson 25 yards out. Up stepped Bouazza, and his resulting free-kick was superbly clawed away at full stretch by home stopper Boaz Myhill.
Former Blue Bouazza conjured up Millwall's next effort, a smart seventh-minute shot on the the turn that flashed wide of the mark.
The Lions had a let-off soon after when Alan Dunne tried to shepherd the ball out but skidded on the grass, enabling Jean Beausejour to nip in and send an inviting delivery across the face of goal that only needed the feintest of touches to convert.
The slippery St Andrews surface proved a frustration for both teams at times, having been heavily watered prior to kick-off, with a number of players lost their footing throughout proceedings.
But there was nothing wqrong with a delightful piece of skill produced by Feeney on the left that came so close to breaking the deadlock for Millwall around the 20-minute mark when his pace and persistence worked an opening on the left flank and he then let fly with a stinging drive that cannoned off the post.
Liam Trotter drilled over the bar after picking up Henderson's neat flick midway through the opening half as Jackett's men continued to ask questions of the home defence.
However with The Blues at this juncture posing little attacking threat, there was a horrible inevitability that their first penetrative attack would produce a goal - and that's exactly what happened.
A quick break from deep in their own half involving the impressive Morgaro Gomis and then Beasejour ended with the latter's low centre being side-footed home from close range by New Zealander Wood.
Curtis Davies went close to doubling Birmingham's lead six minutes before the break, but the big defender sent a downward header a yard wide from Beausejour's corner.
At the other end Trotter's deft control and dinked delivery was a whisker away from dropping onto the head of the onrushing Simpson as the game approached half-time.
Millwall had a great opportunity to level within 60 seconds of the restart when Feeney threaded the ball through to Simpson, but his snap-shot was smothered by Myhill.
The home keeper was certainly the busier than his Millwall counterpart at this stage and Myhill came to the rescue once again, blocking at close range again from Bouazza, who had been played in on goal by Feeney.
However slowly but surely the home side started to get a grip on proceedings with Chris Burke's effort blocked by Tony Craig and keeper Forde pushing Liam Ridgewell's stooping header round the post for a corner.
Lions boss Jackett introduced James Henry and Dany N'Guessan for Feeney and Bouazza, but within seconds of their introduction Wood put the home side 2-0 ahead with a crisp, angled finish that whistled into the bottom corner.
Jackett used his third and final roll of the dice with 18 minutes remaining, sticking John Marquis up front alongside Henderson at the expense of Simpson.
As hard as the visiting fans tried to lift their side, it wasn't until the 86th-minute that Millwall went close to reducing the arrears. A quick free-kick by Trotter just inside the Blues half picked out Marquis, he laid a perfefctly weighted return pass to Trotter who burst into the area before letting fly with a stinging shot that Myhill brilliantly parried.
A goal at that stage could have made for an interesting finale, but as it turned out, Wood had other ideas and put the result beyond doubt in the dying seconds of normal time.
Another quick, slick break down the right saw Burke sprint forward, take a quick look up and pick out young striker Wood with an inch perfect delivery that he headed home with aplomb.
Birmingham City: Myhill, Carr, Caldwell, Ridgewell, Burke, Beausejour, Gomis (Elliott, 77mins), Rooney (King, 64mins), Spector, Davies, Wood. Subs not used: Doyle, Murphy, Redmond.
Booking: Beausejour (44mins)
Scorer: Wood (28, 62,90mins)
Millwall: Forde, Dunne, Craig, Trotter, Robinson, Ward, Bouazza (Henry, 61mins), Mkandawire, Henderson, Simpson (Marquis, 72mins), Feeney (N'Guessan, 61mins). Subs not used: Mildenhall, Abdou.
Bookings: Feeney (15mins), Ward (86mins)
Ref: M. Dean
Att: 17,901 (away 1,064)