Cobblers have effectively been relegated to Sky Bet League Two with the most cruel, heartbreaking of defeats as George Dobson's 91st minute goal snatched a 1-0 victory for Walsall in Saturday's crunch clash at the Bescot Stadium.
Town aren't officially down but it would take victory over Oldham Athletic in their final game of the season next week and an unrealistic swing of 17 goals, plus other results to go their way, to save them, which essentially means Sky Bet League Two beckons next season.
For 80 minutes of Saturday's game, Town were on top and created far more chances than their hosts as they continued their improvement from last week's performance against Plymouth but, in the end, their thoroughly disappointing campaign was capped off in rather apt fashion.
The best two opportunities of a lively and often frantic opening 45 minutes fell to the Cobblers who were the better team for much of it and would have been frustrated not to lead at the break after being denied by the post and then a goal-line clearance.
The match continued in the same vein in the second period but Walsall found an extra gear in the final 15 minutes and Dobson's sweeping finish in the first minute of injury time consigned the Cobblers - and their terrific, near 2,000-strong following - to defeat and, with it, relegation.
Town will be ruing their earlier misses prior to Dobson's well-taken goal but what they will be ruing more than anything is eight disappointing months that ultimately left them with too much to do, despite a brief revival under caretaker boss Dean Austin in recent weeks.
There were no surprises with regards to Austin's team selection as he stuck by the same side which impressed in beating Plymouth Argyle last week, meaning Sam Hoskins led the line again.
The raucous atmosphere at the Bescot was matched by a breathless start as both teams set their stalls out to attack in a hectic, action-packed opening five-minute spell during which an opening goal could have came at either end.
Walsall threatened first when Jon Guthrie couldn't sort his feet out to convert a deep free-kick before Cobblers were then agonisingly close to taking a crucial lead themselves as Liam Roberts turned Daniel Powell's low drive onto the inside of the post, with the ball fortuitously rebounding back into the goalkeeper's grateful arms.
Back came the visitors through Kieron Morris, whose shot from the edge of the box was blocked by David Buchanan, while Matt Grimes also saw two long-range attempts hacked away by defenders.
This had all came inside a captivating first 10 minutes and though the game did naturally settle down after that, there was still plenty happening for both sides with Matt Crooks, in particular, causing problems down Town's right side.
It was his fine work that carved open another decent chance for the away side when beating his man, driving into the box and then aiming for the far post where defender Kory Roberts cleared away off the line.
The midfielder also saw a penalty appeal waved away as Northampton build up a head of steam towards the end of the half but a breakthrough goal remained elusive, and with both Oldham and Rochale drawing elsewhere, the two teams went into half-time still with everything to play for.
Walsall began the second-half with a little more purpose but it was their visitors who had another presentable chance, Shaun McWilliams slashing wide from 20 yards after Crooks had led a counter-attack.
And Crooks was at the heart of the next close shave for the away team when playing a one-two with Facey and then side-footing over first-time from eight yards.
A rare period of Walsall pressure then saw them twice go close in the space of 90 seconds as Erhun Oztumer got the wrong side of Facey and shot into the side-netting before Richard O'Donnell stood up tall to block from Nicky Devlin.
Neither side would have benefited from a draw and that was reflected in a now open and end-to-end game as the two sets of fans roared their teams forwards heading into the final quarter of the game.
Town could not have come any closer to breaking the deadlock with 14 minutes to go when Grimes' mishit cross sailed over everyone and cannoned onto the post, back onto Roberts, onto the bar and then bounced off the line and away from danger.
Another two minutes had passed when Austin went for broke, throwing on strikers Kevin van Veen and Chris Long and moving to 4-3-3, and the former's impact was nearly instant as he controlled a high ball into the box before being unable to get a shot off.
Those changes from Austin seemed to stunt Town's momentum, however, and instead it was Walsall who then pushed for the precious winning goal with Oztumer blazing wide from 20 yards and Devlin heading off target when well-placed.
Ash Taylor was sent up front such was Northampton's desperation for a goal but there was to be a cruel sting in the tail when the Saddlers countered and, three-on-three, the ball was squared for Dobson who swept first-time into the bottom corner, sparking wild scenes among the home fans including a pitch invasion.
Town frantically surged forward in what remained of stoppage-time but no dramatic equaliser was forthcoming and the full-time whistle effectively confirmed their relegation, leaving both players and fans crestfallen and putting the final nail into a miserable campaign.
Walsall: L Roberts, K Roberts, Fitzwater, Guthrie, Leahy, Devlin, Chambers, Dobson, Morris, Oztumer (Cuvelier 90), Bakayoko
Subs not used: Gillespie, Kinsella, Flanagan, Kouhyar, Ngoy, Candlin
Cobblers: O'Donnell, Facey, Taylor, Barnett, Buchanan, Crooks (van Veen 78), McWilliams (Long 78), Grimes, Powell, O'Toole, Hoskins
Subs not used: Cornell, Moloney, Turnbull, Foley, Bridge
Referee: Seb Stockbridge
Cobblers fans: 1,993