Substitute Simon Richman struck in the dying seconds as Port Vale piled more misery on struggling Grimsby.
The teenager fired past Phil Barnes following a neat Louis Dodds flick-on from Steve Thompson's cross.
Grimsby thought they had snatched a point on 85 minutes when Adam Proudlock headed in Jamie Clarke's centre.
But Vale, who earlier took the lead when Marc Richards smashed home from 10 yards, took maximum points when Richman notched in the 88th minute.
Port Vale:
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Goals: Richards 28, Richman 88.
Grimsby: Barnes, Stockdale, Bennett, Hope, Atkinson, Clarke, Kalala, Trotter, Boshell, Jarman (Bore 79), Proudlock. Subs Not Used: Heywood, Llewellyn, Montgomery, Hegarty.
Goals: Proudlock 85.
Att: 5,058
Ref: Karl Evans (Greater Manchester).
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