Town held up their end of the bargain by beating Barrow 3-1 at Holker Street but, incredibly, Bristol Rovers scored five times in the second-half to beat bottom side Scunthorpe 7-0 and overturn the five-goal deficit they faced at the start of the day.
Northampton and Rovers finished level on both points and goal difference but Joey Barton’s side will go up due to the fact they scored more goals over the course of the season. Town will face Mansfield Town in the play-offs, and they will be without goalkeeper Liam Roberts for the first leg after a late red card.
It all looked to be going perfectly against Barrow as Northampton stormed into a 3-0 lead with just 21 minutes gone Sam Hoskins scored twice and Fraser Horsfall headed in another.
But Barrow pulled one back and then Rovers racked up the goals against already-relegated Scunthorpe United, who fielded a very young team in that fixture, much to the dismay of Cobblers supporters.
There was a delay to confirmation as the game at the Memorial Stadium was suspended due to a pitch invasion but belatedly the full-time whistle ensured Cobblers must make do with the play-offs.
It was a crushing and almost unbelievable way for the season to end and somehow Jon Brady must now pick up his players with a second bite of the cherry to come in the play-offs.
Brady was forced into one change as Aaron McGowan missed out with injury and Joseph Mills came in to skipper the side.
Mills started on the right side of a back four and Cobblers had a great early chance but Louis Appere couldn’t get the ball under control from Josh Eppiah’s through pass.
No matter, barely four minutes were on the clock when the away side did hit the front. Eppiah won back possession just outside the penalty box and the ball broke to Hoskins who curled a low shot into the bottom corner for the perfect start.
Barrow initially responded well and embarked on a couple of dangerous attacks, but they fell further behind just 10 minutes later. It was that classic combination of Pinnock’s left foot and Horsfall’s head as the Cobblers centre-half was left completely unmarked to nod home at the back post.
And two became three before a quarter of the game had even been played. Eppiah drove into the box and squared for Hoskins who passed into the bottom corner with a first-time finish.
Liam Roberts made a couple of routine stops but Cobblers continued to threaten and only a terrific defensive challenge denying Eppiah a fourth goal.
Phil Brown made changes in both personnel and formation before half-time and his side improved significantly in the closing stages of the half, grabbing one goal back through Josh Kay’s near post header from a John Rooney.
Cobblers led by two at half-time and with Rovers also ahead by the same margin against Scunthorpe, Brady’s side were in a good position with 45 minutes of the season to go.
But there was still a lot of football to be played and Barrow came out with the bit between their teeth at the start of the second-half, so nearly scoring again when Josh Gordon’s header cannoned off the crossbar.
Rovers scored twice in a few moments to go 4-0 up and things were becoming decidedly edgy for Town, especially as they were still struggling to gain much of a foothold in the second-half at Holker Street.
News then filtered through of Bristol Rovers scoring three goals in quick succession to crush Cobblers dreams.
Suddenly Northampton’s focus switched and they went all out for another goal. They did have a couple of chances to score a fourth goal but Idris Kanu headed wide and Hoskins was denied by Paul Farman
Just to rub salt into the wounds, Roberts was sent off in stoppage-time for handling the ball outside the area and will now be suspended for the play-offs.
Danny Rose went in goal for the final few minutes but it didn't really matter as the full-time whistle confirmed the most disappointing of 3-1 victories.
Barrow: Farman, Brown (Jones 80), Platt, Hutton (Williams 70), Canavan, Stevens (Glennon 31), Grayson, Gotts, Rooney ©, Kay, Gordon
Subs not used: Lillis, Taylor, White
Cobblers: Roberts, Horsfall, Guthrie, Mills © (Zimba 88), Hoskins, Koiki, McWilliams, Sowerby, Pinnock, Eppiah (Rose 60), Appere (Kanu 80)
Subs not used: Maxted, Harriman, Dyche, Pollock, Kanu
Referee: Ross Joyce
Cobblers fans: 575