The Millers substitute struck with a deadly finish from the edge of the penalty area to send the home fans in the 11,607 crowd into raptures.
Proctor’s late leveller, set up by Michael Smith, ensured the hosts claimed deserved reward from a pulsating derby against a Blades side whose goalkeeper Dean Henderson had been in top form.
The on-loan goalkeeper produced a string of fine saves, his best coming late on when he finger tipped a Semi Ajayi header over the crossbar.
Chris Basham put the visitors ahead for a second time moments later and it looked like Henderson had exacted revenge for last May’s play-off final loss to Rotherham when sporting the colours of Shrewsbury Town.
Proctor, however, had other ideas to ensure the spoils were shared at the end of a derby clash that had seen the Blades go ahead inside eight minutes.
A crisp finish from Mark Duffy had left Marek Rodak with no chance in the home goal as Ajayi was punished for a poor clearance that went straight to the visitors.
Rotherham levelled midway through the second half via a Jon Taylor that took a deflection to wrong-foot Henderson.
At that stage, the Millers looked most likely to go on and win but Henderson’s save from Ajayi paved the way for Basham to restore the visitors’ lead from a Billy Sharp cross.
That seemed to be that only for Proctor to come off the bench and ensure honours deservedly ended even in the lunchtime kick-off.
Afterwards, Blades boss Chris Wilder was critical of his players' performance.
"The performance was very, very poor but I need to compliment the opposition manager and their players," said Wilder. "If I was in their changing room, not getting the win, I would be disappointed.
"It has not been a good day for us and I won't hide behind the result. We made four or five mistakes for their second goal which contrived to make sure we didn't win the game."
Millers boss Paul Warne felt a point was the least his side deserved.
"To get the equaliser late on made it feel like a win," said Warne. "We were always more than in the game. We caused them problems. If we had lost the game 2-1 I would still have been hugely proud of the performance.
"We deserved a draw and the lads will take a lot of confidence from it. It feels like a mini win."