Chris Basham's late header looked to have won the game for the Blades, who would have gone back to the top of the Championship table with a victory in the day's early kick off at the New York Stadium.
But substitute Proctor pounced on some slack defending from the visitors to equalise in injury time, and send the home supporters - who had earlier seen Jon Taylor's deflected shot cancel out Mark Duffy's opener for the Blades - wild.
United, who named an unchanged team from their last outing against Wednesday before the international break, began the game on the back foot as home skipper Will Vaulks' volley tested Dean Henderson early on before Vaulks smashed an attempt high into the Blades fans behind Henderson's goal.
But they went ahead as early as the eighth minute when John Fleck seized upon a poor pass out from the back by Semi Ajeyi, Enda Stevens fed Duffy and he applied a calm finish.
Initially it appeared that the United forward had tried to do too much, cutting back on the outside of the recovering Millers defender, but his poise and prescence of mind saw him put the Blades in front.
For a period, that goal lifted United and caught the home side a little off-guard, but Paul Warne's men rediscovered the ascendancy and created further good openings for Michael Smith and Richie Towell, with Taylor particularly effective.
Henderson, who was beaten three times by Millers' Richard Wood last seaspm while on loan at Shrewsbury, made a superb save to tip over Ajeyi's header and then divert Taylor's curling effort wide.
But after the hosts continued to threaten after the restart, their equaliser eventually came - even if it did have a touch of fortune about it. Moments after Henderson had denied Clark Robertson, Taylor ensured he was eventually beaten - albeit with the help of a big deflection, which sent the shot into Henderson's bottom-left corner.
Henderson was at his best later on to deny Ajeyi again, after the giant centre-half rose highest to meet another corner from the Millers left, before the Blades went ahead again when Chris Basham headed home Billy Sharp's deep cross from the left hand side.
But there was still time for another twist to the tale, sub Proctor netting in injury time to level the scores, earn his side a point and prevent United from returning to the top of the table.
Millers: Rodak, Vyner (Vassell, 87), Robertson, Ajayi, Mattock, Williams (Proctor, 87), Manning (Newell, 63), Vaulks, Taylor, Towell, Smith. Not used: Price, Wood, Palmer, Wiles.
Blades: Henderson, Freeman, Basham, Egan, O'Connell, Stevens, Fleck, Norwood, Duffy (Stearman, 87), McGoldrick (Clarke, 62), Sharp (Johnson, 90). Not used: Moore, Baldock, Coutts, Washington.