Full-time from Wembley Stadium
A quick-fire early double from Ashley Fletcher (2mins) and Adam Hammill (19mins) cracker put The Tykes in control and although Mark Beevers gave Millwall a glimmer of hope with a 34th-minute strike, Lloyd Isgrove put the seal on victory with a third and killer goal 16 minutes from time.
Lady Luck certainly was not shining on Millwall and manager Neil Harris was forced to change his starting XI after handing in the team sheet when defender Byron Webster aggravated an injury in the pre-match warm-up, handing a return for club captain Tony Craig, making his first start since 31st October 2015.
For Webster, it was a cruel finale to a season that had seen the centre half go from strength to strength and form a rock solid partnership in the heart of The Lions' back four. Pretty much like the game itself proved to be for The Lions - a bitter pill to swallow after a magnificent effort to get this far.
For Craig, whose last appearance at Wembley for Millwall in the 2010 Play-Off Final against Swindon was cut short when he fractured his foot in the opening half, it was an opportunity to try and take full advantage of an unexpected opportunity.
Craig led the team out holding special mascot for the day Harvey Brown to a rousing reception from around 30,000 Lions fans, who within the opening 30 seconds were on their feet when Steve Morison headed a Chris Taylor free-kick narrowly wide of the target.
Those same supporters were stunned less than 60 seconds later when, with their first attack of the game, Sam Winnall flicked on a long forward ball to Fletcher, who shrugged off a challenge and dispatched a low shot into the bottom corner beyond the despairing reach of Jordan Archer.
It could have been 2-0 on five minutes when Hammill worked an opening on the left, only to thrash his shot wildly over the bar.
Fletcher caused an anxious moment in the Millwall defence after 13 minutes when he latched onto a hopeful forward punt, the flag stayed down, but the angle was too tight and keeper Archer did well to block the angled drive for a corner.
However a second Barnsley goal was not long coming - and it duly arrived with 19 minutes on the clock. Hammill picked up the ball on the left, eased past Carlos Edwards and then Ben Thompson before letting fly from 25 yards with an unstoppable shot that left Archer grasping at thin air.
Matters looked even worse for the shell-shocked Lions moments later with George Williams' crude challenge on Taylor earning the Barnsley man a yellow card and leaving the Millwall winger requiring treatment.
Thankfully Taylor was fit enough to continue and slowly but surely Millwall began to put their nightmare start behind them and start asking questions of their own as Shane Ferguson and Taylor started stretching and really working the full-backs.
A deep ball from the left was headed narrowly wide by Morison and then Ferguson's inviting delivery was gathered under pressure by Adam Davies in the Barnsley goal as Harris' men tried to get a foothold back in the game.
That is exactly what they did 11 minutes before the break. Ferguson's corner was headed across goal by Morison and although Craig tried to connect, Beevers picked up the loose ball and expertly swivelled before slamming a left-foot shot into the back of the net.
No sooner had they reduced the arrears than Harris was forced to reshuffle his defensive playing pack again two minutes before the break. This time it was the unlucky Martin who hobbled off injured. Aiden O'Brien came on to take the left midfield berth vacated by Ferguson, who slotted in at left-back.
Neither manager made any changes at the break at what was the start of a monumental, but ultimately heartbreaking, second-half effort from Harris' men.
Lee Gregory, who had enjoyed little change from Marc Roberts, was a whisker away from connecting six yards out with the first chance after the break after Jimmy Abdou's back header into the mix had caught the Barnsley defence napping.
Gregory was on the end of a crude 62nd-minute challenge that saw Roberts become the second Tyke in referee Stuart Attwell's book, although Taylor's resulting free-kick from a good position on the left disappointingly dropped straight into the hands of keeper Davies.
At the other end Ferguson did well to toe the ball away for a corner with Isgrove looking to reach Hammill's cross and whilst The Lions dealt with that subsequent delivery, Barnsley's next corner saw Isgrove squeeze in ahead of Archer and meet Conor Hourihane's inch perfect delivery to plant a header into the back of the net and restore Barnsley's two-goal cushion.
Harris responded by bringing on Fred Onyedinma and then Shaun Williams for Ben Thompson and Edwards respectively, but for all their effort, overall Millwall struggled to produce the fluid attacking play that had served them so well, so often, this season.
Taylor did see a late shot superbly saved by Davies and Morison's header was straight into the keeper's hands but it simply was not to be Millwalls's day.
The final whistle brought the curtain down on the season, but when the dust of a hugely disappointing afternoon has settled, there is so much to be proud of as the pride, passion and positivity have been restored thanks to Harris and his players over the past 10 months. Roll on next season.
Booking: Williams (28mins)
Scorers: Fletcher (2mins), Hammill (19mins), Isgrove (74mins)
Millwall: Archer, Edwards (Williams, 81mins), Martin (O'Brien, 43mins), Beevers, Craig, Abdou, Thompson (Onyedinma, 75mins), Ferguson, Gregory, Morison, Taylor. Subs not used: Forde, Philpot, Upson, Nelson.
Scorer: Beevers (34mins)
Ref: S. Attwell
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