The Cobblers started the day in pole position to go up, and did their bit as they claimed a 3-1 win at Barrow.
But they were denied a third-place finish as rivals Bristol Rovers almost unbelievably claimed a 7-0 home win over Scunthorpe to seal promotion.
It means that Jon Brady’s men finish the season fourth, and must now take part in the play-offs.
They will play Mansfield Town over two legs to try and reach the Wembley final.
Close to 600 Cobblers fans made the trip to Cumbria, while more than 2,000 turned up at Sixfields to watch the action on a big srceen.
Tings couldn’t have started much better as Town raced into a 3-0 lead, but pretty soon cheers turned to tears.
At half-time, the Cobblers led 3-1 while Rovers were 2-0 ahead, meaning Brady’s team still had a five-goal advantage in goal difference.
But Rovers scored five second-half goals to wipe that out, and with the Cobblers failing to find a fourth, it means Joey Barton’s team will be playing in league one next season.
Go to the Sport section for all the reaction.