A goal in each half from Danish striker Poul Hubertz, a third from Paul Robinson and a late penalty gave Millwall all three points at Griffin Park.
The powerful hit-man gave The Lions a lead inside 23 seconds, added a second just before the hour mark and created the third for his captain moments later.
Bees skipper Kevin O'Connor pulled one back from the penalty spot but Darren Byfield completed the scoring from the spot after Charlie Ide was dismissed for bringing down Chris Hackett.
The game could not have started in a worse fashion for Brentford when they presented the visitors with a late Christmas gift.
Adam Griffiths headed a long ball over the advancing Clark Masters and Hubertz slotted in from a tight angle with less than 30 seconds played.
But despite the dreadful start Brentford held their own in the early stages and by half time could have been at least level.
Half chances fell to O'Connor, Calum Willock and John Mousinho but Lenny Pidgeley in the Millwall goal wasn't tested.
The first clear sight of goal fell to Simon Cox when Jo Kuffour nipped the ball past Pidgeley, Willock was forced to turn and feed Cox who blasted the ball over with the goal gaping.
Ide wasted an opportunity by shooting into the side netting when he could have squared it to the unmarked Cox and O'Connor had a shot deflected narrowly over from a drilled Cox corner.
Hubertz could have doubled Millwall's advantage before the break but shot wide and Masters prevented Byfield scoring a second soon after half time.
In the first 15 minutes of the second period The Bees again had chances to get back into the game.
Danny Senda prevented a drilled Willock cross reaching Ide and O'Connor curled a free kick inches over.
The Brentford skipper should have scored when he blasted over from ten yards after fine play from Cox and Millwall made The Bees pay.
Senda found space on the left, cut back and curled the ball to the near post where Hubertz flicked it in.
Ide could have pulled one back but Pidgeley blocked his fierce shot and just past the hour Hubertz was released by a quick Alan Dunne free kick, his cross was powered home by Robinson.
Dunne produced a wonderful late tackle at the other end to prevent Kuffour scoring and Brentford's top scorer shot wide soon after.
The Bees did pull one back when Willock won the ball deep in Millwall territory and Richard Shaw was forced to foul Ide as he tried to wriggle free in the box.
O'Connor whacked the ball straight down the middle and past Pidgeley as the goalkeeper dived out of the way.
Ide volleyed over as Brentford strove for a way back into the game but he was soon taking an early bath when he pulled down Hackett as he raced free.
Byfield slotted the penalty home with aplomb to complete another bad day for Brentford.
Brentford: Masters; Osborne, Heywood, Griffiths, Frampton; Ide, Mousinho, O'Connor, Kuffour; Willock, Cox (sub Moore 63 mins)
Subs (not used): Brown, Charles, Pinault, Peters
Bookings: Cox (16 mins) (third of season), Osborne (66 mins) (fifth of season) (suspended for one match versus AFC Bournemouth, January 6), Griffiths (85 mins) (eighth of season)
Sent Off: Ide (83 mins)
Millwall: Pidgeley; Dunne, Shaw, Robinson, Senda; Hackett, Elliott (sub Lee 87 mins), Ardley (sub Fuseini 56 mins), Zebroski (sub Williams h/t); Byfield, Hubertz
Subs (not used): Day, Morais
Bookings: Dunne (12 mins)
Written by Chris Wickham