Sam Ackroyd's late strike snatched an important win for United's Under 18s on a bizarre afternoon at the Steelphalt Academy which saw two crazy goals in what was a more than eventful second half.
The first half didn't see a lot of action but Derek Geary's side were largely in control despite a lack of clear chances for both teams.
Leo Gaxha, brought into the team in place of Ackroyd, missed the biggest chance for the Blades after just 10 minutes when Jake Kelly played him in, but one on one he was denied by the opposition goalkeeper.
Marcus Dewhurst was called into action to make a good save from Millwall striker George Alexander and also tipped over a stinging effort from Jayden Davies.
But it was in the second half when the game really livened up and just five minutes after the break it was United who took the lead.
As the Millwall goalkeeper went to kick the ball out of his hands, he slipped and promptly dropped the ball at the feet of Ompreon who was left with a simple tap-in to score the easiest goal he could have wished for.
Unfortunately for United, Milwall were level ten minutes later and it was another strange incident. George Alexander's free-kick beat Dewhurst but bounced down and after a couple of the seconds the officials awarded the goal to the visitors.
In the seconds after the decision, goalkeeper Dewhurst was sent from the field for his reaction to the decision and United had to play out the final half an hour with ten men.
Despite that, United were still more than a match for Millwall as both teams looked for a winner in the closing minutes.
Fortunately, that winner came for the Blades with just two minutes left on the clock when substitute Ackroyd ran through and slotted home a low finish into the corner to steal the three points for United.
Blades: Dewhurst, Williams, Foulstone, Neal, Weaver, Belehouan, J. Kelly (Ackroyd 80), Broadbent, Gaxha (Stainrod 68), Brunt, Ompreon (Chapman 64).
Unused: Clarke, Gordon.