Substitute Asa Hall scored a dramatic 89th-minute winner to keep alive bottom club Luton's faint survival hopes.
Ryan Bennett's looping header into the top corner put Mike Newell's Grimsby on course to win against his old club.
Mark Bower then nodded Luton level in first-half injury time, followed by a scrappy second half.
But Hall volleyed in from eight yards after Michael Spillane's cross had fallen into his path to move the Hatters within 12 points of safety.
Luton:
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Brill, Spillane, Pilkington, Bower (Hall 60), Emanuel, Keane, Nicholls, Jarvis (Henderson 74), Martin (Parkin 83), Craddock, Gallen. Subs Not Used: Button, Davis.
Booked: Bower, Nicholls, Craddock.
Goals: Bower 45, Hall 89.
Grimsby: Henderson, Stockdale, Bennett, Atkinson, Widdowson, Llewellyn, Hunt (North 90), Hegarty, Bore (Heywood 88), Akpa Akpro, Forbes. Subs Not Used: Proudlock, Clarke, Normington.
Booked: Akpa Akpro, Hegarty, Bore, Henderson.
Goals: Bennett 43.
Att: 5,830
Ref: Russell Booth (Nottinghamshire).
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