Second half goals from Johnny Ertl and a penalty from Daniel Bogdanovic was enough to see off Ronnie Moore's men infront of just over 4,000 supporters.
The Millers started the game with Danny Harrison in midfield in place of Jason Taylor and Tom Elliott missed out in attack with a tweaked hamstring, being replaced by Marcus Marshall against a strong Blades line up including two trialists in Shaleum Logan and Gregory Vignal.
The men from Sheffield 2 started brightly forcing a series of corners, but Marshall was the first to have a meaningful effort at goal, with a shot from 15 yards that was blocked by Andy Taylor.
Bogdanovic then coasted through for the Blades on 17 minutes but his cross shot from an acute angle went harmlessly out of danger, and then moments later he was denied after an unlucky bounce in the area alllowed Nick Fenton to make a timely tackle.
The Millers major first half threat came from Adam Le Fondre as you'd expect. The striker was first to pounce on 38 minutes after Steve Simonsen tipped away Kevin Ellison's header. As the ball bobbled across the goaline Alf's effort from the rebound was somehow scrambled to safety.
Simonsen was then at full stretch to tip away another shot from Le Fondre after a foul on Mark Bradley just before the interval.
The travelling Blades fans appeared to have taken a liking to the olympic flame holder too, as they made the most of the afternoon on the grass banking at the away end!
The Blades took the initiative after the break despite some more good interplay from the Millers, as Le Fondre again went close to breaking the deadlock.
Jamie Ward's shot was parried from 30 yards by Andy Warrington and Bogdanovic had a goal chalked off for offside, tapping in after Warrington had saved Ertl's effort from range.
But Ertl wasn't to be denied as he headed in Vignal's 73rd minute cross.
And the win was wrapped up five minutes later as Bogdanovic scored from the spot following Fenton's trip on him.
Teams: Rotherham: Warrington; Green (Ashworth 64), Cresswell, Fenton,
Newey, Law, Harrison (J. Taylor HT), Bradley, Ellison, Marshall (Warne 64), Le Fondre. Subs not used: Annerson, Brogan, Darley.
Sheffield United: Simonsen; Logan (Gilbert HT), Morgan, Nosworthy, Lowton (James 84), Ertl, Montgomery (Britton 64), Taylor (Vignal HT), Yeates (Chappell 67), Bogdanovic, Ward (Foster 75) Subs not used: Aksalu, Cresswell.
Crowd: 4,152 (2533 away)