A night of derby glory for the Millers ... on-the-whistle report: Sheffield United 0 Rotherham United 1
Ben Wiles' first-half goal settled the encounter and the visitors might have won by more had they not been denied what looked like a clear spot-kick close to the interval.
Rotherham, who defended heroically at times, deserved their famous victory against opponents who began proceedings in third place.
It was their first win in five matches and so, so worth the wait.
The game began quietly and it took until the 21st minute for either side to register an attempt on goal.
Even then it was a weak one as Oliver McBurnie's miscued shot was easily dealt with by Viktor Johansson.
Three minutes later the Millers came close to taking the lead when electric pace down the left from Cohen Bramall was followed by a dangerous cross.
Peter Kioso got on the end of it and it took a flying save from Wes Foderingham to tip away the header for a corner.
The match was a sell-out and the away fans were in fine voice as Rotherham gave the home team few sights of goal in the opening half an hour and looked a threat on the counter-attack.
McBurnie looped a 35th-minute effort on to the roof of the net but it was no more than a half-chance.
Soon after the Millers were head as Chiedozie Ogbene cut in from the right and fed Ben Wiles who coolly picked his spot from 12 yards.
Rotherham hadn't won on Blades soil since 1980 but they were daring to dream as the travelling fans upped the volume to a new level.
Oliver Norwood fired a free-kick over the bar before Wiles wasn't far away from doubling his side's advantage as he tried his luck only to lift the ball too high.
There was yet more refereeing controversy attached to a Millers match as Wiles tumbled in the box under the weight of Enda Stevens' presence as he looked certain to score from Ollie Rathbone's pass.
Keith Stroud turned down the penalty appeal and found himself surrounded by livid away players when he blew for half-time a few seconds later.
Rotherham had made four changes to the side that had been beaten by Norwich City at the weekend.
Lee Peltier and Cameron Humphreys came into the back three, wing-back Bramall returned on the left after serving a one-match ban and fit-again Ogbene started up front.
Two minutes after the restart Johansson saved superbly from John Egan to keep the Millers in front.
The hour mark had come and gone when Ogbene made contract with a Dan Barlaser free-kick right in front of goal but could only divert the ball wide with his chest.
Blades pressure was mounting and McBurnie's header was clutched by Johansson who then saved similarly from Chris Basham.
Yet still the Millers led and still their opponents were struggling to create clear-cut opportunities against a strong defence.
Billy Sharp glanced a header wide for the Blades as the clock ticked down and that was the last time the Rotherham net was threatened.
The final whistle brought joy on the pitch and chaos in that away end.
A beautiful end to a beautiful night.
Sheffield United (3-5-2): Wes Foderingham; Anel Ahmedhodzic (Chris Basham 68), John Egan, Jack Robinson; George Baldock, John Fleck (James McAtee 20, Reda Khadra 80), Oliver Norwood, Ben Osborn, Enda Stevens (Billy Sharp 68); Iliman Ndiaye, Oliver McBurnie. Subs not used: Adam Davies, Ciaran Clark, Oliver Arblaster.
Rotherham (3-5-1-1): Viktor Johansson; Lee Peltier, Cameron Humpheys, Wes Harding; Peter Kioso (Brooke Norton-Cuffy 78), Jamie Lindsay ((Tom Eaves 89), Dan Barlaser, Ollie Rathbone (Hakeem Odoffin 89), Cohen Bramall; Ben Wiles; Chiedozie Ogbene. Subs not used: Josh Vickers, Richard Wood, Scott High, Hamish Douglas.
Goals: Wiles 38 (Rotherham).
Referee: Keith Stroud (Hampshire).