The young Lions left it late at Calmont Road as Kristian Brymora struck with one minute left to seal a 2-1 win over South London neighbours Crystal Palace U23s.
Both sides were locked at 1-1 until the 89th minute before Brymora swept home with an instinctive first-time finish to secure victory for the hosts.
Kevin Nugent's men were bolstered by the addition of a number of regular first-teamers, including Ryan Tunnicliffe, Jake Cooper and Ben Thompson, but it was Millwall scholar Mason Saunders who opened the scoring.
The winger slipped in behind the defence in the 13th minute and fired beyond the Palace 'keeper's gloves from close range.
Half-time brought a chance to regroup and the visitors quickly found an equaliser when Andre Coker combined with midfielder Sean Robertson.
But just when Palace expected to head home with a draw, Brymora settled the contest with almost the game's final kick.