It took a 90th minute penalty to end United's unbeaten start to the season.
At the scene of United's last defeat in 90 minutes, back in April, referee Darren Sheldrake penalised David McAllister for a foul on Angelo Balanta and Shaun Williams converted the spot-kick.
For 89 minutes the Blades gave another solid and committed performance before the all-important penalty award.
Chief Wilson, back at another of his former clubs, named an unchanged side. He kept faith with the starting XI that earned a draw at Swindon - the only change was on the bench where young striker Ironside replaced loanee Cofie.
The opening 15 minutes were eventful. On three minutes a neat passing move from the home side resulted in Long at full stretch as Chadwick's effort flew just over the crossbar and then a Lowe centre evaded a sliding Bowditch. For United, Porter won more than his fair share of headers in the opening minutes, including one to allow Flynn to beat Lewington but the subsequent pull back was cleared.
Long saved from Balanta after the winger had beaten McMahon and then the Blades goalkeeper saved impressively from Lowe, not knowing that a linesman's flag had been raised. Porter and Doyle battled in the box from a rehearsed corner, which included Williams and Flynn, and Blackman tried his luck from distance but was off target.
The Dons perhaps shaded the first half hour and the Blades were forced into a change when McMahon pulled up holding his hamstring right in front of the home bench - he was unable to continue and his place was taken by Westlake.
Blackman forced a save out of Martin with a sharp right-footer and the United striker's deft pass almost gave Flynn an inviting shooting chance. Back at the other end, just before the break, Balanta worked hard to deliver a cross from the left but Lowe's flicked header was wide, whilst as injury-time approached the home fans were appealing for a penalty when Collins challenged with Lowe but referee Sheldrake said no.
Hill, who looked to be holding his thigh late in the first half, failed to return for the second period and was replaced by McAllister, which forced a little reshuffle - Williams dropped back to left-back. The first chance after the interval fell at the feet at Lowe in an ideal position but his shot was straight at Long and United hit back with long-range efforts from Doyle and McDonald, both of which were deflected.
A succession of corners for home side failed to break the deadlock whilst the referee again turned down penalty shouts, this time from Unitedites after Blackman and Otsemobor went down.
Wilson used his last substitute with just over 15 minutes remaining, Miller replacing Blackman, and Maguire picked up United's first booking for sliding in on Potter after a heavy touch - from the resulting free-kick the home side's Williams fired over.
The Dons continued to see more of the ball as the game entered the final 10 minutes and Balanta fired over after cutting in from the left before heavily contributing to the home side's cause as the watch ticked around to 90 minutes. From Bowditch's cross, Balanta was heading away from goal when he was adjudged to have been pulled down by McAllister. The referee pointed to the spot and after issuing a booking for protests, it was set for WILLIAMS
to slot home the penalty, although Long went the right way.
As the game restarted three minutes of injury time was announced and even though Collins was added to the front line the Blades had too little time to get back on level terms.
Dons: Martin, Otsemobor, Lewington, S. Williams, Gleeson, Potter, Bowditch, Lowe (Smith 59), Kay, Chadwick, Balanta (Kolo-Doumbe 90).
Unused: MacDonald, Chicksen, McLoughlin, O'Shea, Sekajja.
Blades: Long, Maguire, Hill (McAllister 45), Flynn, Doyle, Porter, Blackman (Miller 74), McDonald, Collins, McMahon (Westlake 26), M. Williams.
Unused: Cresswell, Ironside, Kennedy, Willis.