Millwall increased the pressure on under-fire West Bromwich Albion boss Valerien Ismael as the Baggies' troubles on the road continued with defeat at The Den.
After a drab first half, Millwall were comfortably the better side after the break and Mason Bennett tucked home a Scott Malone free-kick to put them ahead.
Benik Afobe guided a classy second into the bottom corner as the furious away following vented their frustration at Ismael following a third successive away defeat.
Former England striker Andy Carroll, who joined Albion on Friday on a deal until the end of the season, made an encouraging debut but was denied an early opener by Bartosz Bialkowski as the Baggies failed to score for a fourth straight away game.