Mihailo Ivanovic provided a moment of quality in a bruising encounter at Luton to provide Alex Neil with his first win as Millwall manager.
The Serbia international striker volleyed home the only goal to bring the Lions just their second away win of the season and avenge Luton's win at The Den in September - the Hatters' only triumph on the road.
In midweek, Neil was cruelly denied his first victory since taking the reins last month, as Cardiff scored a stoppage-time equaliser.
It looked like his bad luck was continuing as Aaron Connolly won and then missed a first-half penalty, but Ivanovic came to the rescue in a game low on quality.