JOSE Morais is still looking for his second win as Barnsley manager.
The Reds stuttered again at Oakwell, and are now without a win on home turf in their last 11 attempts, a run stretching back to early November.
The play-off chasing Lions grabbed a well-deserved sixth win in their last seven games.
The Reds opened the game impressively enough, but again missed chances blighted their efforts.
Oli McBurnie saw a strike deflected wide following some smart approach play from Mamadou Thiam.
The Senegalese front-man was again the provider shortly after when he crossed for Kieffer Moore, only for his fellow striker to nod straight at grateful Lions 'keeper Jordan Archer.
Those missed opportunities were punished by Millwall in the 24th minute.
Lee Gregory opened the scoring, sliding the ball home as he darted in to meet a low cross from Jed Wallace.
It was almost 2-0 shortly before the break, but Millwall midfielder George Saville blazed over wastefully from a terrific position.
The Reds came out fighting after the restart, and defender Liam Lindsay saw a low drive tipped superbly round the post by Archer.
Moore then leapt highest as he headed just over the crossbar.
The Lions roared two clear in the 63rd minute.
This time wide man Ben Marshall smashed home clinically from 25 yards.
Gary Gardner missed the Reds' best late chance to grab a consolation goal.
Barnsley (4-3-3): Townsend; Yiadom, Jackson, Lindsay (Isgrove, 81), Pinillos; Potts (Bradshaw, 66), Gardner, Mallan (Moncur, 46); Thiam, Moore, McBurnie.
Subs not used: Walton, Cavare, Mahoney, Pearson.
Millwall (4-4-2): Archer; Meredith, Hutchinson, Williams, Romeo; Marshall (Onyedinma, 79), Cooper, Saville, Wallace; Morison, Gregory (Shackell, 75).
Subs not used: Martin, Cahill, Tunnicliffe, Elliott, O'Brien.
Referee: Peter Bankes (Merseyside).