There were 93 minutes on the clock, the last minute of time added on, when substitute Grant Leadbitter lofted a ball into the area from the left and Liam Miller rose to glance the deftest of headers across keeper Stephen Bywater and into the far corner.
The 36,000 crowd exploded with joy after a vital win which leaves Roy Keane's side just three points from the top after a match which looked as though it was doomed to end in a draw.
That would have been hard lines on a Sunderland side which had been full value for their lead going into the break, the home team converting only one of half a dozen great chances in the opening 45 minutes when David Connolly was fouled in the area by Marc Edworthy and steered home from the penalty spot in the 27th minute.
Sunderland had gone 63 games without being awarded a penalty, but Connolly showed no nerves as he shot low to the keeper's right.
Derby came out of the blocks in the second half like a steam train and showed why they were leaders with a non-stop charge on the Sunderland goal which finally bore fruit on the hour with a superb equaliser from sub Giles Barnes, who danced his way through several tackles before driving a low shot home.
Both sides fought all the way to the final whistle but the game looked set to end in stalemate until Miller's last-gasp winner.
* FULL REPORT IN TONIGHT'S FOOTBALL ECHO