Jermaine Easter marked a memorable Millwall debut with his first goal for the club - but then saw red in injury-time when he was sent off for allegedly elbowing Swindon's Jack Smith.
Striker Easter, signed on an emergency loan from Plymouth Argyle on Thursday, made an instant impact for Kenny Jackett's side as scored the opener to send The Lions on their way to a fifth successive league win.
Smith levelled for the home side early in the second-half, only for Lewis Grabban to pop up and hammer home his fourth goal of the campaign 15 minutes from time to clinch a thoroughly deserved victory.
Although Millwall had to play the final two minutes of injury-time with 10 men following Easter's controversial injury-time dismissal, the result of this match was never really in any doubt as Kenny Jackett's men bossed proceedings from virtually the first whistle to the last.
Jackett made a couple of changes to the team that started against Cheltenham last weekend, with Easter going straight into the starting line-up alongside Gary Alexander. Andy Frampton stepped in for Zak Whitbread, who dropped to the bench after suffering from illness.
Zak's absence saw Frampton slot in at left-back and Tony Craig move across to a central defensive role alongside Paul Robinson.
The last encounter between these two teams on the final day of the 2007/8 season was still very much in the thoughts of Den boss Jackett, who had demanded an improved performance from his side following their previous lacklustre display at the County Ground.
It had been a miserable day all round on that last visit to the west country, with Danny Senda's early goal the only positive on an otherwise forgettable afternoon that saw the right-back stretchered off with cruciate knee ligament damage and Millwall return to London pointless following a 2-1 defeat.
But the manager's pre-match call for a repeat of the sort of display that had seen his men record four successive wins going into this game was answered perfectly as The Lions made it five wins out of five to stay in the League One play-off positions with a hard fought success.
Millwall made their intentions clear from the start. Straight from the kick off Dave Martin set off on a strong run down the left and somehow squeezed in a cross that Gary Alexander headed straight at Phil Smith in the home goal.
Martin then capitalised on a poor defensive pass to slip the ball through to Easter, but the debutant's cross was too close to keeper Smith, who collected comfortably at the near post.
Having been denied their first corner of the game on 10 minutes through the inattentiveness of the linesman, The Lions then racked up two in quick succession after 13 minutes, neither of which really tested the home defence.
But there was nothing keeper Smith could do 11 minutes later to prevent Jackett's men taking a well deserved lead on 24 minutes.
Chris Hackett found Grabban and the latter set off on a superb run down the right, timed his delivery to perfection and there was Easter to control the ball before sweeping it into the back of the net.
It could have been 2-0 within three minutes when a short Ali Fuseini free-kick to Grabban saw the striker's piledriver blocked, the loose ball fell to Martin but his fierce drive flew across goal and a couple of yards wide.
Millwall were in total control at this stage with Grabban's shot charged down for a corner, Tony Craig sending a header wide and then lashing over from a Hackett corner as Jackett's men kept their foot firmly on the gas.
Swindon's relief at hearing the half-time whistle gave manager Maurice Malpas an opportunity to try and improve on what had been a fairly shambolic show from his players at that stage of the game.
And whatever the Robins boss said certainly had an immediate impact as it took just 16 second-half seconds for the home side to level when Smith's dipping cross deceived the visiting defence and sneaked in at the far post.
Millwall twice went close to regaining the lead that they had worked so hard to establish and Paul Robinson's 51st-minute header was tipped over by Smith for a corner.
Midfielder Fuseini was then a yard away from scoring the goal of his career with a spectacular first-time strike from the half-way line that dropped agonisingly wide with keeper Smith a hapless bystander.
There was an anxious moment in the visiting goal when Anthony McNamee slipped a ball through to Craig Easton, but his shot was held by David Forde.
A great ball from Hackett saw Alexander and Grabban get in each other's way six yards out enabling keeper Smith to collect the ball as The Lions pressed forward again.
When he was handed an opportunity 14 minutes from time, however, Grabban made no mistake reacting smartly to smash the ball into the roof of the net from two yards after Alexander's header from a Hackett corner had smacked the woodwork.
Easter could have made it 3-1 five minutes from time when he raced clear of a static home rearguard, but hit a shot straight at Robins keeper Smith.
Swindon, who were as ineffective for long periods in the second half as they had been in the first, somehow nearly contrived an equaliser in the 90th-minute when sub Lee Peacock's flicked header was brilliantly tipped over the bar by David Forde in the Millwall goal.
That appeared to be just a minor blip with The Lions in total control of proceedings. But when Easter, who had been booked on 87 minutes for an innocuous challenge on Smith, challenged for the ball with the same player on the right, referee Mr Penn brandished a straight red card to the dismay and amazement of the Millwall camp.
It was a real case of deja vu. At Leicester two weeks ago Tresor Kandol was dismissed in injury-time and The Lions fought out the final couple of minutes to hold on for victory.
Jackett's side did exactly the same again, simply re-grouping, shutting up shot and securing three precious points that no-one could doubt they totally deserved with a thoroughly good afternoon's shift.

Swindon Town: P. Smith, J. Smith, Aljofree, Easton, McGovern (Peacock, 89mins), McNamee, Amankwaah, Nalis (Marshall, 76mins), Paynter, Morrison, Cox. Subs not used: Brezeovan, Timlin, Casal.
Bookings: Nalis (73mins), Cox (88mins)
Scorer: J.Smith (46mins)

Millwall: Forde, Hackett, Frampton, Robinson, Craig, Fuseini (Laird, 74mins), Abdou, Martin, Alexander, Easter, Grabban. Subs not used: Edwards, Whitbread, Grimes, Moore.
Booking: Easter (87mins)
Sending off: Easter (90+1mins)
Scorers: Easter (24mins), Grabban (76mins)

Ref: A. Penn