Bristol City went top of the second division with a well-deserved victory at Grimsby, coupled with previous leaders Plymouth's defeat at Tranmere.
It took a late goal from Aaron Brown to clinch the point three minutes from time after Grimsby had failed to clear a Brian Tinnion free-kick.
And it was an earlier set-piece that proved Grimsby's undoing when a Brown corner was powerfully headed home by impressive striker Lee Miller with ten minutes on the clock.
But Grimsby, to their credit, against a classy team who deserve to win automatic promotion, fought back well.
After on-loan striker Isaiah Rankin was denied a penalty for what looked a clear foul, he did win Grimsby a spot-kick on the half hour when he was brought down by Matt Hill.
Midfielder Iain Anderson confidently dispatched the penalty into the bottom corner to put the Mariners level.
City came out strongly for the second half and Miller chipped a delightful shot against the bar before Danny Coles was denied by a great save by keeper Aidan Davison.
But Grimsby refused to bow the knee and skipper Stuart Campbell hit the post after Grimsby's best move of the game.
But just when it seemed as if they had ridden out the heavy City pressure, the visitors struck late on to go top and send their supporters wild with delight.
After the euphoria of Grimsby's 2-1 win over Brighton on Saturday, which was their first success in 11 games, this was a bit of a set-back although not totally unexpected.
But they will not meet any better side in the run-in and if they continue to show this sort of spirit, they should avoid a second successive relegation.