The Millers were on course to all-but secure at least second spot in the table before Michael Ihiekwe turned the ball into his own net late on after the defender had put Paul Warne's side ahead at the Stadium of Light.
Given that Wigan Athletic lost 3-2 at Portsmouth, the Millers would have moved top of the table on goal difference with a victory in Sunderland.
Warne spoke of his side's devastation after the game but their destiny is still in their own hands as they head to relegation-threatened Gillingham at 12.30pm on Saturday.
The Millers simply need to match or better MK Dons' result this weekend to secure second spot, with MK travelling to play-off chasing Plymouth Argyle in their final fixture.
If Rotherham draw or lose and MK Dons win, the Millers will drop to third and will have to go through the play-offs to earn promotion.
Wigan travel to Shrewsbury Town knowing that their promotion spot is all-but safe given their superior goal difference to MK Dons.
The Millers can still go up as champions as they sit two points behind Wigan with the exact same goal difference. That means that any dropped points for Athletic, coupled with a Rotherham win, would see the Millers lift the League One title.
Whatever happens, it is set to be a thrilling finale in a dramatic campaign.