The Reds have only earned one win under Portuguese head coach Jose Morais, that coming against a Birmingham side that drew level with the Reds on Saturday courtesy of a thumping 3-0 win over Hull City.
Lee Gregory opened the scoring in the lunchtime kick-off with a cool finish in the 24th minute, before Marshall’s curling effort from long range in the 63rd minute sealed the win.
Barnsley boss Jose Morais, whose side have eight games to save themselves, said: “I believe it is disappointing.
“Not only for me, but for the players.
“The reason why is that you prepare a game in a stable way, you come into the game in a strong way, and then in a situation you give the ball away and give away a chance.
“I also believe that when you make a mistake like this you need to recover and react. We took some time to react.
“We pushed the opponents. We tried and we created. We had chances to do it but we didn’t score.
“In front of goal we weren’t effective.
“We believed that we could score.
“We couldn’t do it and they had a good strike, it was a fantastic strike. I just felt disappointed.
“The game started to become more difficult because they are a good side.
“We are not getting the results we need to have.”
Barnsley, who have won just once in their last 12 League games, never looked like taking points from the game.
Kieffer Moore went close on a couple of occasions but was met by an in-form Jordan Archer in the visitors’ goal.
The Reds’ best effort came from Oli McBurnie with around 15 minutes left but his effort was hooked off the line by Jake Cooper to prevent the hosts from getting back into the match.
Barnsley: Townsend, Yiadom, Jackson, Lindsay (Isgrove 81), Pinillos, Mallan (Moncur 46), Gardner, Potts (Bradshaw 66), Thiam, Moore, McBurnie. Unused substitutes: Cavare, Mahoney, Pearson, Walton.
Millwall: Archer, Romeo, Hutchinson, Cooper, Meredith, Wallace, Williams, Saville, Marshall (Onyedinma 79), Morison (Cahill 90), Gregory (Shackell 75). Unused substitutes: Martin, Tunnicliffe, Elliott, O’Brien.
Referee: P Bankes (Merseyside).