There was more good news as Doug Loft completed forty five minutes in his first game since the end of August.
Tariq Issa scored on forty eight and seventy to make it 3-1 and Chris Regis put the icing on the cake with a fourth right on the final whistle.
Dexter Peter equalised six minutes later, and the scores remained level going into the break.
It was a deserved victory for the young side, who went behind when Eze Ebuzoeme headed home on thirty five minutes.
After a cagey start from both sides, the U’s were the first to create a chance of note as Charley Edge headed wide from a Decarrey Sheriff cross on nine minutes.
That was quickly followed by another headed chance, this time Issa failing to find the target from a perfectly placed cross by George Brown.
The Millwall forward line had been starved of the ball in the first quarter of an hour, but Noel Leighton did then get free in the channel to hit a low shot straight at James Bransgrove at his near post.
That effort gave the Lions a bit of a boost and they were on the offensive for the next fifteen minutes.
First, Kyron Farrell hit a free kick over the bar and then Ryan Green twice failed to hit the target with headers, the second when well placed to score.
They were rewarded on thirty five minutes, when Lewis White crossed for Eze Ebuzoeme to score the opener.
The lead only lasted six minutes though, as Dexter Peter towered above the Millwall defence to head home from Charley Edge’s free kick on the left.
With seconds to go in the first half, the final chance fell to Leighton but he overran his touch and couldn’t find the target with a weak shot whilst sliding on the floor to reach the ball.
Loft was replaced at half time by Chris Regis, a change planned due to it being Loft’s first involvement following a long lay-off with an Achilles injury.
Regis had only been on the pitch for three minutes when the U’s went ahead, with Issa heading home into the bottom corner from a right wing cross.
The U’s were buoyed by the goal and Issa and Sheriff combined on fifty two minutes to set up Brown, but his shot was well saved by Sandford in the visitors’ goal.
On the hour mark, Josh Pollard was replaced by Michael O’Donoghue but the change didn’t disrupt the U’s too much.
An Issa cross was headed back by Brown at the far post and Millwall unconvincingly cleared the ball out for a corner.
That came on sixty nine minutes, and a minute later the U’s deservedly extended their lead.
Brown, who was causing problems for the Millwall left back, got in down the right again and cut the ball back for Issa to sweep home to make it 3-1.
The U’s then had to make an enforced change, with O’Donoghue having to make way for Ollie Hensdale after taking a heavy whack in the face.
Millwall did get a late chance to reduce the arrears but Ebuzoeme headed wide from a free kick on eighty nine minutes.
That wasn’t the end of the chances or the scoring though, as Chris Regis drilled a shot into the bottom corner with the last kick of the game.
The final whistle went before the Lions could restart and it was a deserved victory for the young U’s.