Sean O'Brien scored the only goal of the game as Millwall's Under 23s were 1-0 winners over Peterborough United at The Den on Friday afternoon.
The substitute tapped home at the back post on the hour-mark to seal three Professional Development League points for Kevin Nugent's side.
Both Ryan Leonard and Connor Mahoney started against The Posh, with Arthur Penney returning on loan from Welling United to take his place in the defence and Besart Topalloj captaining the side.
With just three minutes played, Millwall would have taken the lead if not for a smart double save by William Lakin. As Zak Lovelace showed strength to win the ball back on the touchline and cross, the stopper first denied Leonard and then Mahoney on the follow-up.
Lovelace, Hayden Muller and Penney all had half-chances as The Lions attempted to open the scoring, with Leonard then whistling a half-volley past the near post as half-time came into sight, but Millwall were dealt a blow shortly before the break as Lovelace had to withdraw through injury - O'Brien his replacement.
On the hour-mark, The Lions took the lead. Topalloj won the ball back on The Lions' left and sent a pass over the top to Seb Drozd - a half-time substitute - who battled to keep possession and cross, to where an unmarked O'Brien was waiting at the back post for a simple tap-in.
Three minutes later, a quick counter-attack nearly doubled The Lions' lead, but after Mahoney played a one-two with Abdul Abdulmalik, the ball was just taken off the former as he shaped to shoot from inside the box.
Jordan Gillmore had to stop from Kai Corbett as the last 20 minutes came into view, with Lakin doing the same at the other end from O'Brien on 76 minutes - and with no further action to report, The Lions claimed all three points.
Millwall: Gillmore, Muller, Topalloj, Trialist A (Allen 79'), Penney, Trialist B, Davis (Drozd 46'), Leonard, Lovelace (O'Brien 42'), Abdulmalik, Mahoney. Subs not used: Walker, Hammond.
Scorer: O'Brien 60'