John Pemberton's Academy youngsters booked their place in the fifth round of the FA Youth Cup with an impressive performance on Tuesday night.
Jordan Slew had a hand in all three goals as the Blades got the better of Millwall at Bramall Lane.
Joe Ironside's first half strike set the ball rolling, whilst an own goal from Connor McLaren made it two. Substitute Connor Martin added a late third to set up a fifth round tie with either Blackpool or Birmingham City, who play on Wednesday evening.
United's side included Long, Harriott and Slew, all of whom have experienced the senior squad, whilst Millwall skipper Gallagher has also had a sniff of the first team at the New Den.
The opening 30 minutes of the encounter was a cagey affair as two evenly-matched teams worked each other out. Half-chances fell to Millwall's Adedipe and United's Slew and Harriott, but neither goalkeeper was worked.
On 33 minutes, the pace of Slew got the 523 fans inside the Lane excited. He escaped his marker and brought Ironside and Gregory into play before McFadzean fired a shot over the crossbar.
Skipper Whitehouse then blasted wide after Kennedy had knocked down a McFadzean corner before United's spell of pressure was rewarded with the opening goal with 38 minutes gone. Slew found space to drill in a low effort, which goalkeeper Farmer could only parry into the feet of IRONSIDE, who stayed onside and calmly slotted into the empty net.
The Blades remained on top until the interval but could not add to the narrow lead and Adedipe failed to level the scores after a mazy run at the start of the second period.
Millwall's half then went downhill in the space of four minutes. On 59 minutes, from Slew's right-wing cross, McLAREN turned into his own net with United players ready to tuck the ball home at the back post and then just after the hour, the Lions lost their captain when midfielder Gallagher was sent off for a late challenge on Harriott.
Millwall coach Scott Fitzgerald - a Blade for a short spell in the 1990s - made a couple of personnel changes but the visitors were unable to get back into the game and Slew and Whitehouse went close to adding to the advantage.
With 15 minutes to go, Millwall's night was summed when Adedipe went around Long and appeared to be brought down by the goalkeeper but the referee waved play on.
United boss Pemberton gave substitutes Wilkinson and Martin second half action, whilst schoolboy goalkeeper Willis played the last ten minutes, replacing Long, who was limping slightly after a collision.
Both teams, no doubt eager to impress, played out the game at a frantic pace and that played into United's hands, with a third goal arriving on 87 minutes. Whitehouse found Slew on the right and his cross was bundled over the line from two yards by substitute MARTIN, despite being tightly marked by one of Millwall's central defenders.
The three minutes of added time just delayed the warm applause for the youngsters from the Unitedites who had braved the cold to see the prospects in cup action.
Blades: Long (Willis 82), Montgomery, Anderson (Wilkinson 73), Harriott, Maguire, Kennedy, Gregory, Whitehouse, Ironside (Martin 78), Slew, McFadzean.
Unused: Barry, Hinchliffe.
Millwall: Farmer, McLaren, Richards (Mathias 61), Shitta, Paul, Goodman, Adedipe (Wilkinson 80), Gallagher, French (Sammoutis 63), O'Brien, Kingsmill.
Unused: Bartlett, Fitzgerald.