Gareth Williams grabbed an injury-time equaliser to deny Grimsby a crucial victory in their bid for First Division survival.
The bottom-placed Mariners survived a vicious Forest onslaught before Williams popped up to poke home Marlon Harewood's cross.
Grimsby's Georges Santos tussles with Darren Huckerby
Alan Pouton had put Grimsby ahead two minutes after the break after Paul Groves' 44th-minute equaliser cancelled out Andrew Reid's deflected opener.
Forest, fourth in the table, underlined their credentials as one of the division's brightest attacking sides in the opening stages.
They took the lead on 22 minutes when skipper Ricardo Scimeca rolled a free-kick to Reid and his 30-yard effort took a wicked deflection off Pouton and flew into the net.
It might not have been too pretty to watch but the players deserve credit for the way they fought back
Grimsby struggled to create anything going forward but they silenced the home crowd with the equaliser two minutes before half-time.
The Mariners patiently strung together 14 passes before Darren Mansaram escaped down the left.
His low pull-back was not dealt with by the Forest rearguard, allowing Groves to ghost in and stab the ball past Darren Ward.
Grimsby stunned their hosts again three minutes after the break when Pouton fired them ahead.
John Oster swung over a free-kick from the right wing, Ward did not get enough on his attempted punch clear and Pouton drilled the ball into the unguarded net.
Paul Hart threw top scorer David Johnson on after an hour to try
and add more firepower up front.
I would have settled for a point before the game but when you get so close to winning it is obviously disappointing
Johnson had the best chance to level but his header from
Harewood's driven cross flew narrowly over.
Forest threw everything at Grimsby, but it looked as if the Mariners were going to hold on for a vital victory until a minute into injury time.
Huckerby found Harewood in space on the left side of the box and his cross was turned past Coyne by Williams.
Nottm Forest: Ward, Louis-Jean, Dawson, Hjelde, Brennan,
Thompson, Scimeca, Williams, Reid, Harewood, Huckerby. Subs:
Jess, Roche, Oyen, Johnson, Doig.
Grimsby: Coyne, McDermott, Ford, Santos, Gallimore, Campbell, Groves, Pouton, Richard Hughes, Oster, Mansaram. Subs: Allaway, Chettle, Barnard, Livingstone, Bolder.
Referee: S Tomlin (E Sussex)