The Posh were comprehensively outplayed in the first half but restricted Grimsby to just one opening when striker Danny North fired over from 20 yards.
Peterborough's cause was not helped by constantly giving the ball away and their re-shaped defence struggled to get any form of cohesion.
But the visitors managed to get to half-time on level terms and it was a different story in the second half. Peterborough's players came out early for a warm-up and it paid instant dividends.
Grimsby's minds were still in the dressing room as Peterborough cut them open down the right and George Boyd slid in the opening goal after 38 seconds of the second period.
The Mariners made a double switch and came roaring back twice creating good openings, but first North and then substitute Gary Jones missed great chances.
And the game was well and truly up when a well-worked free-kick saw keeper Phil Barnes fail to gather a high ball and defender Alan White headed in for a fine debut goal.
That effectively ended the contest, and although Grimsby never gave up, there was never any likelihood of a comeback.
After Grimsby's fine run there had been talk of a late challenge for a play-off place, although manager Alan Buckley had always played that possibility down.
On the evidence of this game it is Darren Ferguson's side that looks the better bet to mount a late run for a play-off spot.
They bounced back well from the defeat at Shrewsbury Town and, if they don't win promotion this time, they must be a sound bet to go up automatically next season.
It was Peterborough's eighth away win of the campaign but, with only 18 goals scored on their travels, they have not been particularly prolific.