Geoff Horsfield's first Blades goal proved to be the extra-time winner and clinched a place in the third round of the Carling Cup following a tough night in Milton Keynes.
Earlier, Chris Lucketti and Nicky Law also claimed their first competitive goals for United on the club's first visit the Stadium:mk.
Boss Bryan Robson, who made six changes from the team that beat West Brom on Saturday, saw his side forced to battle for a full 120 minutes before seeing off the League Two promotion candidates.
If Blades supporters spent the opening minutes taking in the new surroundings, they can rest assured that they missed very little action on the pitch - it was the 10th minute before the first chance was created. From Quinn's quick free-kick, Geary delivered from the right and Carney cut in from the left and saw his well-placed header pushed around the post by goalkeeper Abbey.
Soon after, however, the home side were ahead with their first opportunity. Carayol had caused problems and after the Blades could only half clear, Stirling picked up the loose ball, ran at the defence and delivered a perfect cross for BROUGHTON to beat his marker and Kenny before lifting into the roof of the net from close range.
United shrugged off the deficit and went straight on to the attack, Stead fired over from 25 yards before the leveller arrived on 20 minutes after Quinn's free-kick deflected off the wall for a corner. Law delivered from the right and LUCKETTI rose highest to head home, despite Abbey getting his hand to the ball.
The goal quietened the home fans but it was United's last chance for a while. Just before the half-hour mark Stirling caused yet more problems enabling Navarro to curl a 20-yard shot just over the angle of post and crossbar.
The Dons finished the half the more attacking of the two teams and a number of deliveries into the box caused concern, especially McGovern's drilled corner which Diallo directed over. Law and Carney switched wings to try and muster an opportunity, whilst the last noteworthy event of the half was Stead picking up a yellow card following a foul on Diallo.
United re-appeared for the second half well in advance of the hosts and the same XI that started the game lined-up at the kick-off. The Blades had a lot of the early possession, Sharp saw a weak effort roll across the face of goal with Abbey untroubled and then Diallo blocked from Stead.
Sharp was on target on 50 minutes following a neat turn but Abbey saved comfortably and it was noticeable that the home side were on the back foot. United's penalty appeals were waved away when Stead tussled with McGovern but the pressure paid off on 55 minutes.
Carney began the move with an intelligent ball which found Quinn down the left. From the Irishman's cross, Stead's shot was blocked by Abbey but the rebound fell to LAW, who slotted into the bottom corner.
Stead went close again moments later when he beat Murphy and curled just over and the striker must have sensed it wasn't his night when he met Carney's cross unopposed but Abbey tipped over the crossbar.
United's second half dominance continued and a third should have been added on 72 minutes following a quick break out of defence. Leigertwood and Law were involved before Shelton skipped inside his marker and fired at the goalkeeper's legs.
The visitors were then made to pay for the missed chances when the Dons levelled on 78 minutes. Lucketti was adjudged to have fouled Broughton, who was booked moments earlier for persistent offending. McGOVERN struck the central free-kick from 25 yards and a huge deflection steered the ball away from Kenny and into the net.
Dons brought on Wilbraham and he added a physical presence alongside Broughton and the duo's battle with Morgan and Lucketti was interesting. Four minutes of injury time was greeted with a cheer from both sets of fans but the only chance of added moments was a Shelton curler that was tipped over by Abbey, ensuring that extra-time was needed.
Carney had been struggling in the closing stages of the 90 minutes following a collision with Kenny and Tonge was introduced for the start of extra-time - the last of the six permitted substitutes.
Extra-time began slowly but was livened up by Dyer's off target cross which almost caught out Kenny, whilst Lucketti went close with a header from Tonge's cross. Wilbraham was proving a handful and from McGovern's corner, Morgan and Geary combined to block a shot from the striker.
The Blades delivered a number of crosses to no avail but the three-pronged attack, in place since Horsfield replaced Law, paid dividends on 104 minutes. Shelton attacked down the left, cut inside and curled a shot which Abbey spilled and HORSFIELD used his experience to slide the ball home from inside the six-yard box.
United created a number of chances in the early part of the second period. Stead blazed over and then was part of the build up which saw Horsfield shoot at Abbey. Tonge then bustled his way through the midfield and set up Shelton but the striker cut inside before curling over.
The Dons had a spell of pressure but failed to trouble Kenny and United were more than happy to hold onto possession and run down the clock to secure a place in round three.
MK Dons: Abbey, Stirling, Diallo, McGovern, Gallen (Wright 68), Edds (Wilbraham 78), Broughton, Murphy, Navarro, Swailes, Carayol (Dyer 62).
Unused: Gueret, Kelly.
Blades: Kenny, Morgan, Stead, A. Quinn, Carney (Tonge 90), Kilgallon, Leigertwood, Lucketti, Sharp (Shelton 68), Geary, Law (Horsfield 81).
Unused: Bennett, Ashmore.
