Barnsley kept their fifth clean sheet in seven matches and their conviction continues to grow defensively, with Michael Sollbauer continuing to look a very shrewd signing and Mads Andersen backing up last week's performance at QPR with another disciplined performance.
But on a day when Charlton and Luton both won in games which also kicked off early, there was a sense that this was a bit of an opportunity lost for the Reds in a 0-0 stalemate.
In truth, neither side did enough to merit victory and Barnsley now head into Tuesday's Roses match with Blackburn with the realisation that a win and not a point is truly required.
Barnsley's Oakwell return was a quiet one in the opening 45 minutes in the sort of tight spectacle that you often get towards the tail-end of seasons.
Defensively, the hosts were sound enough, give or take a couple of semi-threatening moments from the visitors, but going forward, Barnsley lacked a touch of quality and conviction.
It was Millwall, whose manager Gary Rowett had cajoled them to get back to basics after letting in three goals last weekend against Derby, who had cause to feel the happier, even if they were to rue their luck with comfortably the first period's most dangerous moment on 21 minutes.
Shane Ferguson's left-footed curling free-kick was a beauty and beat Jack Walton all ends up, but fortunately his effort bounced off the underside of the bar before being cleared as the hosts were afforded a major let-off.
At the other end, Barnsley's best opportunity also arrived from a set-piece with Cauley Woodrow's well-struck effort tipped over by one-time Reds loanee Bartosz Bialkowski on 18 minutes.
It was Millwall who produced more pockets of creation with a goalbound effort from the recalled Mason Bennett close in being blocked while alert defending from Michael Sollbauer denied former Reds forward Tom Bradshaw, returned a start on his return to Oakwell in favour of Matt Smith, after a good move involving Murray Wallace and Bennett.
In a bid to be proactive and perhaps liven up Barnsley a little, Gerhard Struber wasted no time in bringing on Conor Chaplin at the interval for the quiet Elliot Simoes and it was a move that few watching Reds would have disagreed with.
It seemed to perk up the Reds and while the conditions worsened by way of teeming rain, Barnsley upped their intensity.
A nice move ended with Mowatt's fierce shot being blocked by Bialkowski as the hosts finally showed signs of assertion and dynamism.
At the other end, Barnsley had to repel a mini-flurry of corners, with Andersen making one key challenge to deny the Lions as they pressed.
Steady at the back, Sollbauer had a chance to impose himself at the other end, but did not get enough purchase on the ball when well placed after a clever flick from Woodrow, with the onrushing Bialkowski claiming comfortably.
Big Matt Smith was thrown into the fray for Millwall, with the realisation for both sides being that one goal would seal a priceless victory in their particular quests at opposing ends of the Championship table.
A curler from an unlikely source in Murray Wallace flew wide, while at the other end, timely defending from Jake Cooper thwarted Chaplin after a neat Reds move involving Ludewig.
Barnsley: Walton; Sollbauer, Andersen, B Williams; Ludewig, Mowatt, Palmer (Halme 84), Ritzmaier (Styles 60); Woodrow (Thomas 75); Brown, Simoes (Chaplin 45). Substitutes unused: Collins, J Williams, Bahre, Schmidt, Oduor.
Millwall: Bialkowski; Hutchinson, Cooper, M Wallace; Romeo, Molumby (Woods 81), S Williams, Ferguson (Skalak 89); J Wallace (Thompson 89); Bennett (Mahoney 81), Bradshaw (Smith 69). Substitutes unused: Steele, Pearce, Mitchell, Leonard.
Referee: J Simpson (Lancs).