AIDEN O'Brien's double gave Millwall the three points at Bramall Lane as the Lions moved up, temporarily, to eighth place in the Sky Bet League One table.
The Irish striker put Neil Harris' side ahead in the 30th minute with a superb header before substitute Jose Baxter eqaulised on 70 minutes.
O'Brien put Millwall back in front when he stabbed home Shane Ferguson's cross at the back post and the away side hung on to make it four games unbeaten.
This was arguably the Lions' most impressive win of the season as they went into the game missing four senior players.
The young side could have gone behind early on when Che Adams hit the post, but they composed themselves admirably and largely kept the Blades at arm's length to send the 870 supporters in the away end home happy.
Chicarito déjà vu
We wrote last week that Jordan Archer was steadily compiling an impressive highlights reel for the end-of-season DVD. O'Brien is doing the same at the other end.
The quality of his first goal immediately brought to mind Javier Hernandez's effort at Stoke City almost five years ago.
Sheffield United seemed to have dodged the danger when George Saville's corner reached Fred Onyedinma at the back post. He teed up Shaun Cummings whose cross was directed into a crowded box.
United had the numbers to deal with it but O'Brien produced a sharp piece of improvisation with a back-header that evaded the reach of Mark Howard and bounced down off the crossbar and over the line. It was a bonus for the travelling Lions fans that it happened right in front of them.
It didn't quite have the same level of audacity as the Manchester United striker's unique effort, but it showcased the Irish forward's technique and ability.
The second goal was more about positional instinct as he arrived at the back post to prod home Ferguson's perfect cross.
O'Brien started on the left of midfield and there is something Ronaldo-esque in his ability to get on the scoresheet from up front or from either wing.
His value will continue to soar if he keeps up this quality of finishing…
Class of 2015
…as will Saville's with his level of midfield authority.
Harris said this week he would like to extend the loan deal and it's easy to see why. The former Chelsea player looks unhurried on the ball, possess fine technique and, for such a young player, he reads the game astutely.
He's also a physical footballer and one tackle early in the second half briefly threatened an eruption as Blades players ran to remonstrate with referee Andy Haines and both benches had their say.
The average age of the midfield Harris picked was 21.5, and the average age of the entire side 24.5. Carlos Edwards' absence significantly lowered it and his replacement, Cummings, needed help from the youngest member of the quartet to dig him out a couple of times in the first half.
Sheffield United's side contained plenty of forward players with pace in the wide areas and Fred Onyedinma had to race back following a quick counter to prevent a certain goal when he headed away at the back post with Cummings slightly out of position.
Harris had obviously stressed to Ben Thompson not to get ahead of the ball, and he and Saville were disciplined in midfield as Millwall kept their shape with United targeting attacks from wide areas.
For all the young players in the side, however, it was a mistake from an older head that lead to the home side's leveller.
Archer's scream as he came for a cross was audible even in the back of the main stand, but Tony Craig decided not to heed it and when he cleared, the ball immediately went back to United. They worked it wide and when Kieron Freeman's cross came in Baxter was there to sweep the ball home.
Man of the match
Not that there weren't other candidates, as Mark Beevers was again solid while Cummings and Joe Martin diligently stuck to their tasks in the full-back positions and the work rate on the whole was clearly evident. Steve Morison also led the line well.
But it had to be O'Brien for his decisive contributions.
He also worked extremely hard to chase back and get blocks in, and he deserved the adulation from the away end when he was replaced.
The fact that Millwall also had Alfie Pavey, Jack Powell and Mahlon Romeo on the bench reflected the rebuilding work based on youth that O'Brien is currently most impressively representing.