Luke Guttridge’s wonder goal secured the victory for the Cobblers that takes them to the npower League Two play-off final at Wembley.
Town ran out 1-0 winners at Whaddon Road thanks to Guttridge’s excellent volley and a Lee Nicholls save from Marlon Pack’s first-half penalty.
It means the club will play Bradford City in the final at the national stadium on Saturday May 18, their first visit to the ground since its redevelopment and the third time they have featured in the final.
Both wins at Cheltenham were deserved ones, and with them the scorers, Guttridge and Roy O’Donovan, have written their names into Sixfields folklore regardless of the outcome.
The game’s first goalmouth action came in the sixth minute when a hooked free-kick was headed into Paul Benson’s path by Steve Elliott, Lee Nicholls comfortably making the save.
From the goal kick, Town mounted an attack that ended with a good chance for Roy O’Donovan, but one Scott Brown repelled with his right foot from his goal line.
Then came Nicholls’ moment of play-off heroism. Ben Tozer took too long to complete a clearance that was charged down by Russell Penn, who advanced into the box with the ball and was brought down by Tozer.
It was a sloppy error but one remedied by Nicholls, who guessed the right way and kept out Marlon Pack’s poor penalty, with Nathan Cameron sweeping up the rebound and clearing to safety.
Nicholls made another impressive - though less spectacular - save with his other hand to palm away a Pack free-kick from fully 35 yards.
His duel with Guttridge was a feature o fboth legs of the contest but the Northampton man took the upper hand with a simply superb volley over Brown to give the visitors the lead.
A short Lee Collins throw was chested into Guttridge’s path by Adebayo Akinfenwa and the midfielder unleashed an instant, unstoppable volley that nestled in the roof of the net as soon as it left his foot.
Hackett then forced Brown into a good, backpedalling save when he almost lazily returned the ball goalwards after it was partially cleared from a long Tozer throw on the right wing.
Cheltenham lacked a spark throughout, notably during the second half, in which their supporters grew increasingly anxious and their play more inaccurate.
Time and time again they created the necessary space in midfield to pick a pass but selected the wrong option or simply overhit the ball they did choose.
Not that any of that bothered Northampton, who, with 20 minutes to go, replaced leading goalscorer Akinfenwa with second leading goalscorer Kelvin Langmead in their forward line.
The game briefly sprang to life with a quarter of an hour to go, substitute Kaid Mohamed heading Michael Hector’s right-wing cross against the bar.
Nicholls had no chance to react to the effort but made a first-rate instinctive stop from Sam Deering’s snap shot moments later, parrying the ball up in the air and then claiming it under pressure.
When it did eventually break in the box - more by accident than design - it fell to Sido Jombati, a left-back who produced a shot that was as harmless as the Cheltenham attack had been all evening.
They did not produce anything as clear-cut as the penalty, the failure to score from which was the game’s turning point.
But take nothing away from the Cobblers, who got their tactics spot on - playing an extra man in midfield - and executed them to near-perfection.
Cheltenham were held at arm’s length for long periods of the contest and, when Northampton needed to produce a piece of match-winning quality, they did so through Guttridge.
He was excellent in the first leg and won the second one. The reward is Wembley.
Cheltenham: Brown, Lowe, Hector, Elliott, Jombati, McGlashan, Pack, Penn, Deering, Duffy, Benson
Subs: Harrad (for Duffy 63), Mohamed (for McGlashan 73), Harrison (for Penn 88)
Not used: Jones, Carter, Roberts, Taylor
Cobblers: Nicholls, Johnson, Carlisle, Cameron, Collins, Hackett, Harding, Guttridge, Tozer, O’Donovan, Akinfenwa
Subs: Langmead (for Akinfenwa 71), Hornby (for Guttridge 75)
Not used: Demontagnac, Platt, Snedker, Robinson, Moult
Cobblers fans: 1,546