Millwall 2 (Onyedinma 27, 82)
Reds 3 (Winnall 37, Mawson 56, Nyatanga 90+2)
Birthday boy Lewin Nyatanga popped up with a celebratory winner in stoppage time as Barnsley enjoyed all the points in the capital.
It was a fairly quiet opening as far as chances were concerned, with Abdou only slicing well wide with an ambitious effort on three minutes.
The Reds enjoyed a decent passage of play on eight minutes with Bree sending over a spectacular cross that Winnall seemed destined to meet at the far post but Nelson flicked away at the last second.
On 12 minutes Marley Watkins forced David Forde into a smart save as Crowley's deep cross found the forward in space. Watkins' volley to the near post had to be turned out for a corner by Forde.
Two minutes later Dan Crowley had a shot at goal himself but saw his placed effort land the wrong side of the post after cutting in from the left.
The home fans' frustrations at the opening 17 minutes were almost eased when Lee Gregory just missed out on a probable goal when Onyedinma's cross from the right just evaded his studs.
On 25 minutes the Reds probably should have opened the scoring. A Very neat counter-attack started by Bree and Pearson saw Crowley thread a ball into the path of Winnall but his shot lacked accuracy just as he raced in from the left-hand corner of the box, scooping over the bar.
When the opening goal did come, it came against the run of play. Somehow Onyedinma managed to dig out a shot whilst laid on the deck following a scramble in the box and his tame effort trickled over the line.
Millwall did have the ball in the net again on 33 minutes but the referee's assistant's flag was up for offside on Onyedinma.
The hosts' lead lasted 10 minutes as Sam Winnall notched his first of the season from barely a yard. Crowley's cross was controlled well by Watkins, whose shot across the face of goal was turned in by Winnall at the far stick.
The Reds were hopeful of starting the second half just as they started the first but the home side registered the first effort on target of the second half. Williams' goal-bound header was cleared off the line by Bree at the far post on 52 minutes.
On 56 minutes the Reds took the lead and it was through an emphatic finish by Alfie Mawson. When a free-kick by Hourihane dropped Mawson's way from Roberts' header, Mawson absolutely lashed into the bottom corner.
Reece Wabara was introduced for his Barnsley debut as he replaced James Bree at right-back two minutes after the Reds' second goal.
Just before the hour, Watkins almost made it 3-1 when he rifled just over the top corner from 22 yards following a great piece of robbery by the Reds on the edge of Millwall's box.
Millwall responded by making a double substitution with the home fans growing increasingly frustrated at the Reds' ability to win back and maintain possession.
Gregory and O'Brien both saw long-range efforts go wide on 74 minutes but at the other Barnsley really had Millwall on the ropes when they countered. Crowley this time dancing into the box before seeing his powerful cross cleared.
Physio Craig Sedgwick had to be called on for Ben Pearson as the midfielder went down following a challenge with Abdou.
The home side were desperately searching for a way back into the match and throwing everything into the box. Roberts was on hand to clear Cummings' dangerous ball on 80 minutes just before Ben Pearson read a ball into the channel well and prevent Onyedinma from creating a chance.
The Reds just couldn't hold onto their lead as, frustratingly, Millwall equalised through a free-header from Onyedinma - heading home an in-swinging corner on 82 minutes.
Barnsley could have made it 3-2 when they again countered to perfection through Watkins. His cross from the right set up a 50-50 between Forde and Rothwell that the goalkeeper managed to win.
With five minutes of added time indicated, the Reds huffed and puffed with Watkins running his blood to water.
His endeavours didn't go unrewarded as when he won a free-kick for a deliberate foul on the right, Hourihane swung in a brilliant cross that birthday boy Lewin Nyatanga rose highest to meet and head into the net in front of the travelling Tykes.
It could have been perfectly wrapped up by Conor Wilkinson when he latched onto Conor Hourihane's through-ball with a blistering strike but Forde denied him with a great save.
Soon after, the final whistle went and it was a deserved victory for the super Reds.
Millwall: Forde, Cummings, Craig, Williams (Thompson 61), Upson (O'Brien 61), Gregory, Onyedinma, Nelson, Ferguson (Martin 90), Morison, Abdou.
Subs: Archer, Cowan-Hall, Marquis, Webster.
Reds: Davies, Bree (Wabara 56), Nyatanga, Roberts, Mawson, Scowen, Hourihane, Pearson (Rothwell 82), Watkins, Crowley, Winnall (Wilkinson 90).
Subs: Townsend, Smith, R. Williams, Digby.
Referee: Dean Whitestone