The Reds bow out of the Emirates FA Cup as the Lions go through to Round Four with convincing win.
Barnsley's FA Cup journey ended as soon as it began this afternoon as Paul Heckingbottom's men suffered for a horror second half where the controversial dismissal of Joe Williams tipped the game in the hosts' favour.
It had been a bright Reds start however and Brad Potts grabbed the opening goal following a fine move from back to front.
Jason McCarthy put a superb ball into the channel which Adam Hammill cleverly crossed first time from the right, the ball deflected into the path of Potts who hammered the ball home from six yards out,
The Reds had further chances, with Tom Bradshaw, Dimitri Cavare and Lloyd Isgrove all seeing efforts going wide as the visitors dominated the first half hour.
However, Barnsley would rue those missed opportunities five minutes later..
A fine run down the left by Jed Wallace and his cut-back was fired in despite Adam Davies getting a touch to the shot from six yards out.
HALF TIME: Millwall 1-1 Barnsley
It was a shocking start to the second half for Hecky's men as they soon found themselves 2-1 down..
On 47 minutes, Ben Thompson fired in after another cross from the wing wasn't cut out.
Five minutes later and the game was effectively killed off as a contest when Williams lunged into a tackle on the far side, taking out Wallace and the referee Mr Bond sent him off.
O'Brien soon made it 3-1 with a rocket of a shot from 25 yards out, before adding their fourth when Onyedinma beat his man and then clipped a shot beyond Davies.
The game petered out afterwards with neither side really looking likely to score.
It was a day to forget for the Reds.
FULL TIME: Millwall 4-1 Barnsley