Goals from Aiden O'Brien (2), Ben Thompson and Fred Onyedinma eased Millwall into Round Four of the Emirates FA Cup on Saturday afternoon at The Den with a 4-1 victory over 10-man Barnsley.
Brad Potts gave The Tykes the lead early on, but a quartet of unanswered strikes from The Lions ensured a safe passage into the next round.
Barnsley had Joe Williams sent off early into the second-half for a crunching tackle on Jed Wallace.
Neil Harris made six changes from Monday's 2-1 defeat at Norwich City, with David Martin - for his first start at The Den - Conor McLaughlin, Tony Craig, Thompson, Wallace and O'Brien replacing Jordan Archer, Mahlon Romeo, James Meredith, Ryan Tunnicliffe, George Saville and Lee Gregory in the starting line-up.
The Lions mounted early pressure and nearly found a way through to goal via Wallace and Onyedinma's crosses, but the Barnsley defence stood firm. Craig sliced Millwall's first attempt wide.
The Tykes looked to have fashioned a chance on goal on nine minutes, but Gary Gardner dragged his shot horribly wide. However, just over a minute later, the visitors took the lead after Potts swept home Adam Hammill's cross.
A goalmouth scrambled ensued with 23 minutes on the clock, but following Shaun Williams' corner-kick, the ball was unable to be turned home and was cleared away to safety.
Tom Bradshaw could have doubled Barnsley's lead two minutes later, but the striker's effort was deflected behind. Long and patient build-up play on 32 minutes nearly carved out an opening for Millwall, but the final ball was pushed behind by The Tykes. From the resulting corner, O'Brien headed wide.
10 minutes from the break, O'Brien equalised for The Lions after some neat passing play involving Wallace and Onyedinma. It was nearly a quick-fire double for the Irishman after Wallace's outstanding pass sent the attacker into a foot race with Barnsley goalkeeper Adam Davies, but after O'Brien flicked over the stopper's head, no Millwall body was in place to tap home into the empty net.
The home side were now on top and could have taken the lead through Wallace, but the winger took too long to get a shot away, with the ball eventually cleared. Wallace once again found himself in front of goal a minute before half-time, but poked his effort wide after beating Davies to the ball.
The second-half was barely 90 seconds old when The Lions took the lead. O'Brien controlled the ball on the right-hand side and crossed to Thompson, who fired home past Davies and grabbed the goal that his performance deserved.
On 50 minutes, Barnsley were reduced to 10 men when Williams crashed into Wallace on the far side and received a straight red card. Millwall's tails were up and they could have had a third shortly after, but Jake Cooper prodded over the crossbar.
The lead was extended on 56 minutes as O'Brien controlled Wallace's shot and steered beyond Davies into the far corner from outside the box. Steve Morison struck wide shortly before the hour-mark as The Lions looked to exploit the man advantage and score a fourth.
That fourth arrived on 61 minutes as Onyedinma found space to latch on to a through-ball and sweep over Davies to seemingly put the game beyond doubt.
Following that flurry of goals the game petered out as a contest somewhat, with George Moncur stinging the palms of Martin in the last real action of the game on 82 minutes.
Millwall: Martin, McLaughlin, Hutchinson, Craig, Williams, Thompson, Onyedinma (Romeo 85'), Wallace (Twardek 75'), Morison (Gregory 72'), O'Brien, Cooper. Subs not used: Archer, Meredith, Tunnicliffe, Saville.
Scorer: O'Brien 35' 56', Thompson 47', Onyedinma 61'
Barnsley: Davies, McCarthy, J. Williams, Lindsay, Hammill (Thiam 64'), Bradshaw, Isgrove (Moncur 64'), Cavare, Pinnock, Potts (Mallan 73'), Gardner. Subs not used: Townsend, Brown, B. Williams, Smith.
Scorer: Potts 11'
Booking: Cavare, Gardner
Sent Off: J. Williams
Referee: D. Bond
Attendance: 5,319 (452)