Trailing 1-0 until the 67th minute Grimsby hit Rochdale with three goals in seven minutes to end the game as a contest.
And then the lively Michael Reddy raced clear in added time when the Rochdale corner broke down and the striker finished the game in style Rochdale crumbled after conceding the equaliser and there was never any chance of a repeat of the famous comebacks against Chester and Shrewsbury in recent matches.
After a quite start the game developed into a good contest and Rochdale carved out a brilliant opening goal on 27 minutes.
Striker Grant Holt set up Rickie Lambert to squeeze the ball in at the far post. The goal sparked Grimsby and they could of been level before the break and giant central defender Robert Jones was desperately unlucky to have a header blocked on the line.
Rochdale opened the second half brightly and Grimsby were struggling to get any grip on the game.
A second Rochdale goal would surely have killed the game off. But the game turned just before the hour mark when sub Gary Cohen replaced Jean-Paul Kalala Kamudimba in midfield.
A great run set up Gary Jones to level things with a simple tap in. Two minutes later Gary Cohen headed in from a corner and on 74 minutes Robert Jones slid in to make it 3 - 1 to Grimsby.
That was it until added time when Ready brought the house down by scoring Grimsby's fourth goal.