A stunning free-kick in the last minute secured all three points for Grimsby against a spirited Forest Green side.
The Mariners went in front when Liam Hearn collected Anthony Elding's knock down to score from close range.
But the visitors soon hit back, Yan Klukowski netting from the spot after Gary Silk's challenge on James Norwood.
Sam Russell saved a late penalty from Elding after Jake Thomson's handball but Conor Townsend's free-kick minutes later left him helpless.
VIEWS FROM THE DRESSING ROOM
Grimsby Town joint-manager Rob Scott told BBC Radio Humberside:
"I won't sit here and say that I didn't believe we'd still get three points right until the last minute when we're asking our players to carry on believing.
"We didn't play particularly well first-half but I thought second-half we were by far the dominant team.
"They looked to break on us on the counter attack but I thought we had the better chances.
"Performance wise we probably weren't at the races after we scored."
Forest Green Rovers boss Dave Hockaday told BBC Gloucestershire:
"It's a harsh way to lose a game. I honestly didn't think it was a free kick but we've given the officials the chance to make the decision. It's a tough way to lose but we have to take it on the chin."
"First half I thought we played very well, in the second half we didn't get going and we invited pressure. They stuck two big lads up top and played a simple game and there is nothing wrong with that. We didn't manage the game well enough in the second half."
"We've not come here to become anybody's patsies and for long stretches in the first half they were chasing shadows. I'm just disappointed we weren't braver in the second half, but if we beat Hayes & Yeading next Saturday we'll be back on track again."