Liam Trotter kept his cool to slot home a dramatic injury-time leveller as The Lions snatched a draw from the jaws of defeat against Southampton.
In an amazing final 60 seconds, prolific Saints striker Rickie Lambert looked to have secured a classic smash and grab raid when his free-kick took a wicked deflection off Trotter to loop over keeper David Forde.
But The Saints failed to concentrate and were caught out straight from the restart, with Trotter powering forward from midfield before despatching a calm angled drive beyond the reach of keeper Kelvin Davis.
Millwall's goal was just reward for a gritty, energetic performance that really deserved more than the one point eventually clinched.
Southampton, who had prepared for the game with a spot of warm weather training in Spain, fielded virtually an entire new back four, with Jose Fonte, Dan Seaborne and Jon Otsemobor all handed their debuts.
Lions boss Kenny Jackett made four changes to the side edged out of the FA Cup at Derby, with Trotter, Danny Schofield, Scott Barron and Alan Dunne replacing Marc Laird, Dave Martin, Andy Frampton and Jack Smith respectively.
There was clearly no hangover following the heartbreak of Tuesday's Cup defeat at Derby, with the lively Lions looking fresher, more inventive and playing with greater impetus than their south coast opponents.
On any other occasion Lewis Grabban could have headed home with the matchball tucked under his arm, but it just seemed to be one of those days when as hard as he tried, he was not going to score.
Chris Hackett produced the first real penetrative Lions attack on 16 minutes with a mazy run that saw him skip past three Saints players before his shot was deflected for a corner..
At the other end Dean Hammond spurned a glorious opportunity to put the visitors ahead when he was picked out by Lloyd James from a corner. Thankfully, the unmarked midfielder somehow contrived to head well wide of the target from eight yards.
Hammond's chance was sandwiched between two for the unfortunate Grabban; firstly when he fired well wide after Steve Morison had slipped the ball into his path and then he struck the ball straight at keeper Davis, who had parried Dunne's 36th-minute 25-yard piledriver.
Grabban was a whisker away from breaking the deadlock early in the second half. Schofield picked out his team-mate with a perfectly weighted sliderule pass on the left, Grabban broke into the area and with keeper Davis commited, the young Lion rolled the ball a foot wide of the target.
A super piece of improvisation from Morison ended with the striker's rasping drive flashing narrowly over and then Jimmy Abdou's shot took a deflection and looped into the hands of Davis as The Lions continued to press.
Millwall were denied a clear penalty on 61 minutes when Schofield was sent sprawling on the edge the area, but referee Andy Woolmer incredibly waved played on.
Schofield was again in the thick of the action a couple of minutes later, picking up another Morison flick-on and letting fly from the edge of the area that was pushed for a corner by keeper Davis at full stretch.
Grabban then blasted over from six yards as Millwall kept banging away relentlessly in a bid to make the all-important breakthrough.
Lions boss Jackett brought on Neil Harris for the final 20 minutes and within seconds of his introduction, Hackett caught Scott Barron's excellent cross from the left on the volley, but his rising effort was well over the bar.
Skipper Paul Robinson then headed wide from a Hackett delivery before Southampton gave a reminder that they were still a threat, thankfully the smart reactions of Forde averted the danger with a brave save at the feet of Morgan Schneiderlin.
Sub Michail Antonio sent a scorcher narrowly wide of the post following a surging forward run whilst Lions sub Harris saw his trademark curler pushed over for corner number nine.
A huge upfield punt by Robinson fell kindly for Harris two minutes from time, but on this occasion Chopper's volley lacked power and precision.
With the game heading towards the fourth minute of injury-time Lambert was rewarded with a free-kick when Robinson was penalised for the softest of fouls 30 yards out.
Lambert, who had been superbly marshalled by Robinson and Darren Ward all afternoon, stepped up and couldn't believe his luck as his free-kick took a huge deflection off Trotter to beat the wrong-footed Forde.
That should have been that, but straight from the restart Morison won yet another aerial battle, Trotter used his strength to power past one player and then showed real quality to work his way past another before despatching a low angled drive beyond the reach of Davis for a most dramatic of equalisers at the end of a real emotional rollercoaster ride.
Millwall: Forde, Dunne, Barron, Abdou, Robinson, Ward, Schofield, Trotter, Morison, Grabban (Harris, 68mins), Hackett. Subs not used: Sullivan, Martin, Craig, Smith, Fuseini, Price.
Booking: Robinson (10mins)
Scorer: Trotter (90+4mins)
Southampton: Davis, Otsemobor, Fonte, Seaborne, Harding, James (Antonio, 73mins), Hammond (Wotton, 37mins), Schneiderlin, Holmes, Lallana (Papa Waigo, 90mins) Lambert. Subs not used: Bialkowski, Thomas, Perry, Connolly.
Bookings: Seaborne (72mins), Forte (79mins)
Scorer: Lambert (90+3mins)
Ref: Andy Woolmer
Att: 11,524 (away 1,929)