Millwall were left counting the cost on an instantly forgettable afternoon that saw two Lions players see red - and Chris Hughton's Blues side smash home six goals without reply.
To say it wasn't Millwall's afternoon was understatement as Kenny Jackett saw Shane Lowry (21mins) and then Alan Dunne (56mins) shown straight red cards, leaving The Lions a goal - and two men - down with 59 minutes on the clock.
Jackett's men had already been trailing to Curtis Davies' 18th-minute opener for The Blues when Lowry was dismissed for an alleged stamp on Nikola Zigic, before Dunne left the field for a high challenge on the same player.
Birmingham - and Marlon King in particular - made the most of their numerical advantage and rattled in five goals, including four in the final 16 minutes.
"It was a bizarre," Kenny Jackett said. "We started very well and created numerous chances but didn't put them away, then we conceded a disappointing goal from a set play. I thought both sendings off were harsh. I don't think there was much in Shane Lowry's and I had a better view of Alan Dunne's and didn't see a red card coming. I will have a look at them and see about appealing."
It had all started so well for The Lions, who came flying out of the traps and caused some early problems for the visiting defence.
None more so than on three minutes, when Hamer Bouazza, the one-time Blues loanee, breezed past Chris Burke and delivered a pinpoint cross onto the head of the unmarked Dany N'Guessan, but his bullet header flew wide from six yards.
However, Birmingham's resilience told, and they promptly took the lead after weathering the early storm.
Winning a free-kick 20 yards inside the Millwall half, Jordan Mutch flighted in a perfect set-piece from the right and picked out unmarked defender Curtis Davies at the back post, who thumped a header past David Forde.
Three minutes later matters went from bad to worse when Aston Villa loanee Lowry was dismissed following an off the ball incident.
Midfielder Jimmy Abdou dropped back alongside skipper Paul Robinson as a makeshift centre half, and the spirited Lions rallied, creating two decent chances to level before the break.
Both fell to Harry Kane who saw one header flash wide of the target and then saw his super 18-yard strike tipped over the bar by the fingers of keeper Boaz Myhill.
Millwall again started the second period as they had the first, looking to find a way back. But any hopes of a stirring fightback were quashed before they even really started once vice-captain Dunne took an early bath that left The Lions down to nine men.
Once King had swivelled on a sixpence and rifled in Birmingham's second goal, there was to be no way back as City made their extra man advantage count.
King fired wide from 20 yards as the Blues looked for further goals and they eventually found a third on the counter attack as the former Gillingham striker slipped a pass in to Burke on 74 minutes and he curled a sublime left-foot shot out of the reach of home keeper David Forde.
Another well-worked move allowed sub Adam Rooney (81mins) to add the gloss with a fourth, before King (83mins), and then sub Nathan Redmond, with the last kick of the game, made it six.
Millwall: Forde, Dunne, Smith, Abdou, Robinson, Lowry, Bouazza, Trotter, Kane (Marquis, 85mins), N'Guessan (McQuoid, 71mins), Feeney (Henry, 85mins). Subs not used: Mildenhall, Craig.
Bookings: Trotter (41mins), N'Guessan (62mins)
Sendings off: Lowry (21mins), Dunne (56mins)
Birmingham City: Myhill, Caldwell, Ridgewell, Burke (Redmond, 81mins), King, Beausejour, Mutch (Gomis, 46mins), Fahey, Zigic (Rooney, 60mins), Spector, Davies. Subs not used: Doyle, Murphy.
Scorers: Davies (18mins), King (59, 82mins), Burke (74mins), Rooney (80mins), Redmond (90+2mins)
Bookings: Mutch (3mins), Zigic (35mins), (Rooney (63mins)
Ref: J. Moss
Att: 10,539 (away 958)