Despite failing to hit the net in their three previous games, Grimsby produced the greater early threat with Kevin James, on loan from Nottingham Forest, injecting plenty of pace up front. He set up Peter Bore as early as the sixth minute but the 18-year-old's first touch let him down and the chance was wasted.
Gary Jones let fly from 12 yards only to see his shot deflected by Mark Arber and cleared by Guy Branston. Then a well positioned James was denied by Dean Holden's superbly timed tackle.
Posh failed to seriously test Phil Barnes at the other end with Branston heading a Jamie Day free kick wide before both Richard Butcher and Danny Crowe were well off target.
The balance of the game looked set to change dramatically on 36 minutes when Jones was shown a straight red card by referee Mike Thorpe for what appeared a fairly innocuous tackle from behind on Peter Gain. Yet it was Grimsby who took control with a 52nd minute goal as midfielder Ciaran Toner, once on loan at Posh, drove a shot through a crowd of players from 10 yards.
Former Grimsby defender Ben Futcher marked his full debut for Posh by using every inch of his 6ft 7in frame to reach a Gain corner and level matters with a downward header on the hour.
However Grimsby went ahead again with the exciting Bore nipping in to intercept a poor Branston back header on 73 minutes, before Benjamin turned to beat Barnes with nine minutes left.
Posh produced a ferocious finish and Benjamin, Crow and substitute Lloyd Opara were all denied by an overworked Barnes.