It was a tough afternoon for U18s on Saturday as injuries both during the week and in the match itself conspired against the young U's.
Saturday's eleven featured a mix of U16s, U18s and triallists and it was the home side who had the first sight of goal as Marley Marshall shot just wide of the target.
Al-Amin Kazeem set up Jake Hutchinson to head over, before the U's number nine also pulled a shot just wide of the target.
However, Millwall got their noses in front on the quarter hour mark as Sam Skeffington centred the ball for Reuben Duncan to convert.
The goal lifted the visitors, who went on to have the better of the first half, and the U's also lost Casey Petitt to injury as he was replaced by George Wind on the half hour mark.
Having seen off several Millwall chances, the U's then went close on the stroke of half time, as Harvey Sayer (pictured) pulled one just wide.
Still in the game at the break, the U's unfortunately fell further behind not long after the restart.
Abdul Abdulmalik sent a ball in from the right which took a deflection and Wind couldn't prevent the ball from going into the back of the net.
Abdulmalik made it three on the hour mark, striking a low shot just inside Callum Coulter's right hand post.
Nine minutes later, the Lions had a penalty and Ben Mitchell sent Coulter the wrong way to stretch the lead to four.
A Wind own goal made it five, before Millwall substitute Sam Edozie struck a shot that was saved by Coulter but span on and into the net.
The scoring was rounded off by Duncan, who got his second and the Lions' seventh as he headed home a corner.
U's: Coulter, Anderson, Cracknell, Massingham, Kazeem, Patino, Petitt, Marshall, Cornish, Sayer, Hutchinson
Used subs: Wind, Stagg, Fouche
Unused subs: Collins, Gray