Romaine Sawyers' 47th-minute strike condemned Millwall to defeat at Griffin Park on Saturday afternoon against Brentford in the Sky Bet Championship.
In a keenly and evenly contested London derby, The Bees' solitary strike was just enough to outmuscle The Lions, who missed a penalty through Lee Gregory.
Manager Neil Harris made two changes from the 3-1 defeat to Barnsley before the international break, with Jake Cooper and Shane Ferguson coming into the side at the expense of the injured Byron Webster and Steve Morison. Meanwhile, Shaun Hutchinson captained Millwall for the first time.
The first chance of the afternoon went Millwall's way in just the second minute, but Jed Wallace's snapshot went wide.
Wallace linked up well with Aiden O'Brien on 13 minutes, but Ryan Tunnicliffe's goal-bound shot was blocked by the Brentford defence after the ball was laid off to the midfielder.
The Lions had a golden chance to take the lead in the 20th minute, but after Gregory took advantage of a mistake in the home team's backline, the striker's shot was just turned wide of the post. From the resulting corner-kick, the unmarked Cooper headed towards goal, but his effort was again turned behind.
Brentford opened their shots on goal account on 24 minutes as Ollie Watkins drove over the crossbar following good work from the impressive Josh Clarke. Two minutes, the Millwall defence failed to close down Sawyers, with the attacker's curling attempt forcing Jordan Archer into a smart save.
The Bees were on top as the half-hour mark approached and should have taken the lead through Neal Maupay - however, the striker blazed over with most of the goal at his mercy.
On 38 minutes, Wallace forced Daniel Bentley into a save at full-stretch from a free-kick as The Lions attempted to gain a foothold once more.
The second-half was barely two minutes old when Brentford took the lead - Sawyers' low effort beat Archer and nestled into the corner of the net.
The Lions could have levelled just seconds after the resumption, but a scramble inside the six-yard box eventually lead to The Bees clearing the ball away from danger.
On 55 minutes, Millwall broke away down the right-hand side and after a neat turn by Conor McLaughlin, a cross into the box was spilled by Bentley. Unfortunately for The Lions, the ball trickled on to the post and back into the arms of the goalkeeper.
As the clock ticked on to the hour-mark, Wallace stung the palms of Bentley with a rasping drive. Shortly before, Tom Elliott made a welcome return to action as he replaced James Meredith. The momentum was with The Lions, but the end product was missing, with O'Brien shooting over.
However, it was to be The Bees that had the next chance on goal, with substitute Lasse Vibe sliding the ball wide after some good build-up play. Sawyers repeated the feat on 71 minutes as Millwall lived dangerously at the back in their search for the equalising goal.
At the other end, though, The Lions were fluid in attack and could have had a penalty-kick after captain Hutchinson went to ground. Referee Lee Probert waved away a smattering of protests.
With just 12 minutes remaining, Gregory expertly took Tunnicliffe's ball down on the edge of the area with his back to goal and spun to shoot, but his attempt was well met by Bentley.
The striker had his chance to exact revenge from the penalty spot on 81 minutes after Wallace wrong-footed his defender and was brought down, but once again the stopper was equal to the striker.
Watkins had the opportunity to make the game safe three minutes from time, but shot over.
The Lions frantically searched for an equaliser in the final stages, but could not find it, instead succumbing to defeat.
Brentford: Bentley, Bjelland, Jozefzoon (McEachran 75'), Yennaris, Maupay (Vibe 65'), Watkins (Mepham 90'), Egan, Woods, Sawyers, Clarke, Dalsgaard. Subs not used: Daniels, Archibald, Barbet, Chatzitheodoridis.
Scorer: Sawyers 47'
Millwall: Archer, McLaughlin, Meredith (Elliott 59'), Hutchinson, Gregory, Ferguson, Wallace, Tunnicliffe, O'Brien (Onyedinma 75'), Saville, Cooper. Subs not used: Martin, Craig, Thompson, Romeo, Twardek.
Referee: Lee Probert