The comeback was complete just shy of the hour mark when Harry Wilson slotted in from the penalty spout after Mason Bennett was brought down.
Leeds made it 3-3 on aggregate not long after, however, as Dallas bagged his second of the night.
The drama was far from over, though, with Gaetano Berardi's sending off preceding Marriott putting Derby into pure ecstasy.
The Derby striker clipped the ball over the advancing Casilla to set up a play-off final showdown with Aston Villa.
What Frank Lampard told Derby stars before stunning comeback win over Leeds (Image: GETTY)
What Frank Lampard told Derby stars before stunning comeback win over Leeds (Image: GETTY)
Not even Scott Malone's late red card could take the gloss off what was a hugely impressive show of resilience.
And Lampard revealed post-match what he had told his Derby team to inspire the comeback.
"I said to the lads that everyone had written us off," Lampard told Sky Sports. "We change the system and made a little tweak.
"But we needed the lads to show bravery on the ball and they did. The goal before half-time gave the players belief and I am so delighted for Jack Marriott.
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