But even had they won Grimsby would have been denied automatic promotion as Leyton Orient held onto to third price by wining at Oxford who dropped out of the league.
Grimsby were denied victory by a goal in the third minute of added time by already- promoted Northampton.
But even had they won, Grimsby would have been denied automatic promotion as Leyton Orient held on to to third place by winning at Oxford, who dropped out of the league.
It was a tense match at Blundell Park with Grimsby taking control in the second half as the Northampton defence showed why they had conceded almost a club record of the lowest number of goals.
When the breakthrough came after 72 minutes, it was from the penalty spot with striker Gary Jones being held down in the box by Luke Chambers.
Substitute JPK Kamudimba showed no nerves as he maintained his 100 per cent record for the season from 12 yards to send the home fans delirious.
With Grimsby buzzing and Northampton clearly having done enough in previous games to go up, it looked like the Mariners would hold out for a precious victory. News of Orient's winner at Oxford, however, combined with Northampton's last gasp equaliser dampened the party mood.
Substitute Ryan Gilligan squeezed the ball home from a corner with Grimsby boss Russell Slade showing his frustration on the bench.
This result means fourth place for Grimsby and a two-legged semi final with county rivals Lincoln City. The first leg at Sincil Bank is next Saturday lunch time with the Mariners out to atone for a 5-0 thrashing in the league which sowed the seeds of doubt for their promotion bid.
But if they can get a good result away from home, the return leg on Tuesday, May 16, could provide a launching pad for a first ever visit to the Millennium Stadium in Cardiff at the final on Saturday, May 27, against either Cheltenham or Wycombe.
For Northampton it was a case of job already well done, although it was nice for them to finish this season on an unbeaten note.