Goals from Alan Connell and Wayne Brown gave Brentford victory and dented Grimsby's play-off hopes.
Danny North put Grimsby ahead when he met Nick Hegarty's cross and steered a glancing header into the net.
Nathan Elder twice went close for Brentford, but Peter Till also threatened for the Mariners before the Bees took control after the break.
Connell headed home Craig Pead's cross to level and Ciaran Toner gave the ball away for Brown to score the winner.
Grimsby: Barnes, Bennett, Fenton, Newey, Clarke, Hegarty, Hunt (Jones 56), Boshell, Till, North (Toner 56), Butler (Taylor 74). Subs Not Used: Montgomery, Atkinson.
Goals: North 19.
Brentford: Simon Brown, Pead, Bennett, Osborne, Dickson, Shakes (Wayne Brown 76), Mousinho, O'Connor, Poole, Elder, Connell (Reid 87). Subs Not Used: Hamer, Sankofa, Heywood.
Booked: Pead.
Goals: Connell 51, Wayne Brown 82.
Att: 4,620
Ref: Scott Mathieson (Cheshire).
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