Aiden O’Brien’s early goal earned Millwall victory as Brentford’s recent away troubles continued.
The match-winner came after just eight minutes, when Bees keeper David Raya was closed down and O’Brien slotted the loose ball into an empty net.
Brentford’s Bryan Mbeumo forced a smart stop from Millwall keeper Bartosz Bialkowski, one of only four shots on target the two sides mustered between them.
Czech teenager Jan Zamburek scuffed wide late on as the Bees’ tried in vain to find an equaliser.
O’Brien had scored in Millwall’s previous two home games, late on against Nottingham Forest and Barnsley, but the Lions had failed to win either.
This time, their solidity and organisation earned them their first clean sheet in five at The Den in a game where neither goalkeeper was stretched too often.
Brentford have now lost three of their past four away league games, and have not won on the road since their 3-0 success at Wigan on 9 November.
However, they end the decade fourth in the Championship – their highest position in the English football pyramid at the end of a calendar year since World War Two.