Millwall took full advantage of ten man Barnsley to romp through 4-1 in the third round of the FA Cup.
The Reds had gone in front at The Den but the hosts fought back to lead before Joe Williams was dismissed and Millwall added another two goals in quick succession.
Barnsley made two changes with Dimitri Cavare playing left-back in favour of Zeki Fryers and Tom Bradshaw starting up front in place of Mamadou Thiam.
Youngsters Will Smith and Ben Williams also made the bench.
Barnsley took an eleventh minute lead with McCarthy finding Adam Hammill whose cross to the back-post found Potts to smash by David Martin.
Bradshaw came close to doubling the advantage but his effort from Lloyd Isgrove's cross was deflected behind.
The hosts came back into the game and levelled through Aidan O'Brien on 35 minutes.
Jed Wallace worked himself into a crossing position and O'Brien was perfectly placed to turn a shot past Adam Davies.
Wallace should have put the hosts in front going into the break but his close-range effort rolled wide of the far post.
Millwall did not have to wait long to go in front after the restart with the unmarked Ben Thompson smashing in from O'Brien's centre.
The Reds' task of getting back into the game was made harder five minutes later with Joe Williams sent off for a lunging tackle on Wallace.
Millwall took advantage of the extra man and went further ahead on 56 minutes through O'Brien's curling effort from the edge of the box.
Fred Onyedinma made it four as he beat Davies one-on-one five minutes later.
Paul Heckingbottom made a trio of substitutions with Thiam, Stevie Mallan and George Moncur brought on but it was Millwall who continued to threaten more with Steve Morison dragging an effort just wide.
Moncur tried his best to get Barnsley back into the game but his effort was easily held by Martin before he fired off target in the final minute.
Millwall: Martin, McLaughlin, Hutchinson, Craig, Williams, Thompson, Onyedinma (Romeo 85), Cooper, Wallace (Twardek 75), Morison (Gregory 72), O'Brien.
Unused: Archer, Meredith, Tunnicliffe, Saville.
Barnsley: Davies, McCarthy, Pinnock, Lindsay, Cavare, Isgrove (Moncur 64), J Williams, Gardner, Potts (Mallan 73), Hammill (Thiam 64), Bradshaw.
Unused: Townsend, Smith, B Williams, Brown.
Referee: Darren Bond (Lancashire).
As it happened:
16:49pm: Full time. Millwall 4, Barnsley 1. Barnsley exit the FA Cup after a second half collapse. Joe Williams was sent off at 2-1 and the hosts added two more goals in nine second half minutes.
16:43pm: 85 - Millwall make their final change with Romeo on for Onyedinma.
16:40pm: 82 - The first shot on target of the half from Barnsley as Moncur stings the hands of Martin.
16:39pm: 81 - Gardner now goes into the book for a late tackle.
16:38pm: 80 - Cavare wins Barnsley a corner with a rampaging run but it comes to nothing.
16:36pm: 78 - Barnsley venture forward with Thiam but his effort is deflected on its way through to Martin.
16:34pm: 75 - Twardek is now brought on for Wallace.
16:31pm: 73 - Barnsley make their final change. Potts off for Mallan.
16:30pm: 72 - Gregory comes on for Morison.
16:29pm: 70 - Thiam is throwing his weight around early on but Barnsley look beaten.
16:22pm: 64 - Double change for Barnsley. Hammill and Isgrove are replaced by Thiam and Moncur.
16:21pm: 63 - A rare chance for Barnsley but Gardner's effort is blocked.
16:19pm: 61 - Millwall 4, Barnsley 1. Onyedinma beats Davies one-on-one. Total collapse in this half from Barnsley.
16:17pm: 59 - Millwall threaten again. This time Morison drags an effort just wide.
16:16pm: 56 - It's O'Brien's second of the day. His effort from the edge of the box was too good for Davies.
16:14pm: 56 - Millwall 3, Barnsley 1.
16:10pm: 51 - It's Williams who has seen red for a foul on Wallace.
16:09pm: 51 - Barnsley down to ten men.
16:07pm: 47 - All too easy for Millwall and the unmarked Thompson smashes them ahead following a low cross.
16:05pm: 47 - Millwall 2, Barnsley 1
16:04pm: 46 - Millwall get the second half underway. Barnsley will attack the end with their supporters in.
16:02pm: The teams are back out at The Den.
15:48pm: Half time. Millwall 1, Barnsley 1.
15:45pm: 45 - Two minutes will be added on.
15:45pm: 45 - Millwall come extremely close to going ahead. Wallace's shot dribbles just wide. Davies is down and needs treatment.
15:41pm: 40 - Cavare picks up a booking for Barnsley.
15:36pm: 36 - O'Brien threatens again and pokes the ball around Davies but can't get it the other side.
15:35pm: 35 - O'Brien was on hand to steer in Wallace's cross.
15:34pm: 35 - Millwall 1, Barnsley 1.
15:28pm: 28 - Wallace strikes straight into the wall and Davies catches the follow-up cross.
15:27pm: 27 - Morison wins Millwall a free-kick right on the edge of the box after nipping ahead of Pinnock.
15:26pm: 26 - Cavare's header from the corner goes behind.
15:25pm: 25 - Isgrove's cross is met by Bradshaw but it's deflected behind for a Barnsley corner.
15:23pm: 23 - Barnsley scramble away a dangerous corner.
15:22pm: 22 - Good defending from Pinnock, against his former club, denies Millwall a chance.
15:14pm: 14 - "It's happening again," and "can we play you every week?" are blasted out by the travelling supporters.
15:11pm: 11 - Hammill's cross to the back post was met by Potts who gave Martin no chance.
15:11pm: 11 - Millwall 0, Barnsley 1. Potts strikes Barnsley in front!
15:09pm: 9 - It's been a quiet start here. Millwall edging it with Onyedinma looking threatening.
15:08pm: 5 - Cavare is playing left-back with McCarthy in his usual right-back position.
15:00pm: 1 - Barnsley get us underway.
14:57pm: The teams are about to walk out at the Den.
14:37pm: It was hardly the kindest FA Cup draw and is the third time in four weeks Barnsley have been in the capital.
14:36pm: It's a cold day in South London. About 500 Barnsley fans have made the journey.
14:14pm: Millwall subs: Archer, Meredith, Romeo, Tunnicliffe, Saville, Twardek, Gregory.
14:14pm: Millwall: Martin, McLaughlin, Hutchinson, Craig, Williams, Thompson, Onyedinma, Wallace, Morison, O'Brien, Cooper.
14:02pm: Youngsters Ben Williams and Will Smith are named on the bench by Paul Heckingbottom.
14:02pm: Barnsley: Davies, Cavare, Pinnock, Lindsay, McCarthy, Isgrove, Williams, Gardner, Potts, Hammill, Bradshaw. Subs: Townsend, Smith, Williams, Mallan, Moncur, Brown, Thiam.
13:45pm: Team news is coming up at 2pm.
13:44pm: Barnsley are expected to be without Andy Yiadom who has an ankle injury. Ryan Hedges, Adam Jackson and Matty Pearson are also still sidelined.
13:33pm: The Reds were knocked out at this stage last season – going down to a replay defeat against Blackpool at Oakwell.
13:32pm: Barnsley are aiming for a sixth win on the trot against The Lions and to make it two in a season at The Den.
13:31pm: Welcome to today’s coverage of Barnsley’s trip to Millwall in the third round of the FA Cup.
