Kaid Mohamed's header hit the bar for Cheltenham but the visitors held on to earn a Wembley date with Bradford City.
The Bantams had already overturned a first-leg deficit
in defeating Burton Albion
in the other League Two play-off semi-final earlier in the day and Cheltenham were hoping to do something similar at Whaddon Road.
Northampton's first-leg hero Roy O'Donovan had the first genuine sight of goal in the second leg, but this time he was less clinical. Having latched onto Adebayo Akinfenwa's accurate pass he was unable to slot the ball beyond goalkeeper Scott Brown, who got the slightest of touches to turn his shot away.
Cheltenham had reached the play-off final on all three previous occasions in which they have participated in the end-of-season showdown, winning promotion to the third tier in 2002 and 2006.
The Robins, unbeaten in 11 matches on their own ground coming into the play-offs, continued to have the better of the general play though and appeared fortunate to be handed a golden opportunity to level the tie.
Tozer's challenge on Penn was reckless and referee Andrew Madley did not hesitate in pointing to the spot, despite replays suggesting that the contact had come outside the penalty area.
The Cobblers would have felt justice was done, however, when Nicholls produced a brilliant one-handed stop to keep out Pack's effort.
Nicholls, on a season-long loan from Wigan, was again Pack's nemesis soon after, this time moving smartly to his right to keep out the 22-year-old's 25-yard free-kick.
With the game continuing to seesaw from one end to another, where Pack had failed from distance at one end Guttridge spectacularly succeeded. His dipping volley gave Brown no chance and with it Northampton took a giant step towards Wembley.
Repeating the trick
Northampton boss Aidy Boothroyd took Watford up to the Premier League in 2006 via the play-off route.
While the first period had been an open affair, albeit with both teams looking most dangerous from set-pieces, the second was scrappier and, aided with a two-goal cushion on aggregate, the visitors looked to be relatively comfortable.
In an attempt to alter the momentum of the tie, Robins manager Mark Yates turned to joint-second top-scorer Mohamed and the substitution almost paid off with immediate effect.
Right-back Sido Jombati got to the byline to deliver a superb cross for his former Bath City team-mate Mohamed and, although Nicholls was at last beaten, his thumping header came crashing off the crossbar and away.
Sensing their opportunity, the home side continued to press and Sam Deering stung the fingertips of Nicholls from close range when he might have done better.
Shaun Harrad was denied by the 20-year-old goalkeeper once more from close range as Cheltenham piled bodies forward, but Northampton played out the closing stages to secure a trip to Wembley.
VIEW FROM THE DRESSING ROOM
Cheltenham Town boss Mark Yates told BBC Radio Gloucestershire:
"They've got the job done on us, but we gave it everything and showed people what we're about.
"We got beaten by a great strike, but we played very well. We had lots of chances. Their keeper's played unbelievably well.
"Marlon Pack has been our best player since January 31st. He takes no blame whatsoever for missing a penalty. He's big enough to stand up and take it in a big game.
"Michael Hector played with a pain-killing injection in his ribs and he was outstanding. But there were lots of big performances. We could still be there until 9 o'clock in the morning and not score."
Cobblers manager Aidy Boothroyd told BBC Radio Northampton:
"Cheltenham put us under the cosh. We didn't play particularly well with the ball, but we were resolute and strong. Clarke Carlisle and Nathan Cameron were immense.
"Sensible players do daft things and that's what Ben Tozer did, unfortunately. He was a bit rash. But Lee Nicholls was outstanding.
"I was very fortunate to work with Ben Foster and I genuinely think Lee is as good as he is.
"I don't want to go to Wembley for a day out. I want to win."
Full Time The referee blows his whistle to end the game.
Michael Hector takes a shot. Save by Lee Nicholls.
Keith Lowe takes a shot. Save by Lee Nicholls.
Centre by Michael Hector, save made by Lee Nicholls.
Shaun Harrad takes a shot. Lee Nicholls makes a save. Corner from right by-line taken by Kaid Mohamed.
Effort from the edge of the box by Sido Jombati goes wide of the right-hand upright.
Substitution Byron Harrison comes on in place of Russell Penn.
Keith Lowe sends in a cross, clearance by Ben Tozer.
Free kick awarded for a foul by Sido Jombati on John Johnson. Free kick taken by Lee Nicholls.
Free kick awarded for a foul by Marlon Pack on Roy O'Donovan. Clarke Carlisle takes the free kick.
The ball is crossed by Sam Deering, clearance made by Nathan Cameron.
Free kick awarded for a foul by Shaun Harrad on Lee Nicholls. Indirect free kick taken by Lee Nicholls.
Sam Deering takes a shot. Save made by Lee Nicholls.
Centre by Sam Deering, clearance by Nathan Cameron.
Michael Hector sends in a cross, Kaid Mohamed takes a shot.
Free kick taken by Marlon Pack.
Substitution Lewis Hornby is brought on as a substitute for Luke Guttridge.
John Johnson challenges Kaid Mohamed unfairly and gives away a free kick.
Substitution Kaid Mohamed on for Jermaine McGlashan.
Inswinging corner taken by Ben Harding.
Direct free kick taken by Marlon Pack.
Substitution Kelvin Langmead is brought on as a substitute for Adebayo Akinfenwa.
Free kick awarded for an unfair challenge on Shaun Harrad by Clarke Carlisle.
Centre by Keith Lowe, Header from close in by Paul Benson misses to the left of the goal.
The ball is delivered by Russell Penn, clearance made by Clarke Carlisle.
Free kick awarded for an unfair challenge on Shaun Harrad by Nathan Cameron. Shot comes in from Marlon Pack from the free kick.
Sido Jombati fouled by Chris Hackett, the ref awards a free kick. Scott Brown takes the free kick.
Substitution Shaun Harrad joins the action as a substitute, replacing Darryl Duffy.
Free kick awarded for an unfair challenge on Steve Elliott by Luke Guttridge. Scott Brown takes the free kick.
The ball is delivered by Keith Lowe.
Sam Deering takes a shot. Nathan Cameron gets a block in.
A cross is delivered by Keith Lowe.
Michael Hector fouled by Roy O'Donovan, the ref awards a free kick. Indirect free kick taken by Keith Lowe.
Free kick awarded for an unfair challenge on Sido Jombati by Chris Hackett. Marlon Pack takes the direct free kick.
The assistant referee flags for offside against Darryl Duffy. Lee Nicholls takes the indirect free kick.
Roy O'Donovan gives away a free kick for an unfair challenge on Keith Lowe. Indirect free kick taken by Keith Lowe.
The ball is crossed by Sam Deering.
Booking Lee Nicholls goes into the book.
Unfair challenge on Ben Tozer by Jermaine McGlashan results in a free kick. Indirect free kick taken by Lee Nicholls.
Unfair challenge on Luke Guttridge by Marlon Pack results in a free kick. Indirect free kick taken by Nathan Cameron.
The referee gets the second half started.
45:00 +2:05 Half time
Half Time It is the end of the first-half.
Jermaine McGlashan fouled by Luke Guttridge, the ref awards a free kick. Marlon Pack takes the free kick.
Darryl Duffy has a shot on goal from long range that goes wide left.
Unfair challenge on Luke Guttridge by Keith Lowe results in a free kick. Ben Tozer restarts play with the free kick.
The ball is crossed by Ben Harding, clearance by Russell Penn.
The ball is crossed by Chris Hackett.
Chris Hackett takes a shot. Save made by Scott Brown. Corner taken short by Chris Hackett.
Paul Benson is caught offside. Free kick taken by Lee Nicholls.
The ball is delivered by Jermaine McGlashan, save made by Lee Nicholls.
Michael Hector takes the indirect free kick.
Booking Luke Guttridge goes into the book for unsporting behaviour.
Foul by Luke Guttridge on Russell Penn, free kick awarded.
Corner taken right-footed by Sam Deering, Keith Lowe takes a shot. Lee Nicholls makes a comfortable save.
Header from deep inside the penalty area by Adebayo Akinfenwa goes harmlessly over the crossbar.
Foul by Sido Jombati on Ben Tozer, free kick awarded. Clarke Carlisle takes the indirect free kick.
Corner taken by Sam Deering, Ben Tozer manages to make a clearance.
Luke Guttridge takes a shot. Save made by Scott Brown.
Free kick awarded for an unfair challenge on Lee Collins by Jermaine McGlashan. Free kick taken by Lee Nicholls.
Centre by Sido Jombati, comfortable save by Lee Nicholls.
Assist by Adebayo Akinfenwa.
27:04 Goal scored
Goal! - Luke Guttridge - Cheltenham 0 - 1 Northamp'n Luke Guttridge gets on the score sheet with a goal from outside the penalty box high into the middle of the goal. Cheltenham 0 (0)-(2) 1 Northampton.
Free kick awarded for an unfair challenge on Lee Collins by Jermaine McGlashan. Lee Nicholls takes the indirect free kick.
Adebayo Akinfenwa challenges Steve Elliott unfairly and gives away a free kick. Scott Brown takes the indirect free kick.
Luke Guttridge gives away a free kick for an unfair challenge on Sam Deering. Indirect free kick taken by Michael Hector.
Corner taken right-footed by Sam Deering, Steve Elliott takes a shot. Save by Lee Nicholls.
Foul by Clarke Carlisle on Paul Benson, free kick awarded. Strike on goal comes in from Marlon Pack from the free kick, save made by Lee Nicholls.
Ben Harding takes a inswinging corner to the near post, clearance by Sido Jombati.
Paul Benson handles the ball and concedes a free kick. Lee Nicholls takes the free kick.
Darryl Duffy has an effort from just inside the box that misses to the left of the goal.
A cross is delivered by Keith Lowe, clearance by Clarke Carlisle.
A save by Lee Nicholls from penalty by Marlon Pack. Cheltenham 0 (0)-(1) 0 Northampton.
Booking Ben Tozer is given a yellow card.
Penalty awarded for an unfair challenge on Russell Penn by Ben Tozer.
The ball is sent over by Sam Deering, Clarke Carlisle makes a clearance.
Foul by Luke Guttridge on Marlon Pack, free kick awarded. Indirect free kick taken by Keith Lowe.
Nathan Cameron gives away a free kick for an unfair challenge on Paul Benson. Marlon Pack restarts play with the free kick.
Russell Penn challenges Ben Harding unfairly and gives away a free kick. Lee Nicholls takes the free kick.
Free kick awarded for a foul by Roy O'Donovan on Jermaine McGlashan. Indirect free kick taken by Scott Brown.
Roy O'Donovan takes a shot. Save by Scott Brown. Inswinging corner taken by Chris Hackett from the left by-line.
Paul Benson takes a shot. Save made by Lee Nicholls.
Free kick awarded for a foul by John Johnson on Russell Penn. Marlon Pack takes the direct free kick.
The referee blows for offside. Indirect free kick taken by Lee Nicholls.
Chris Hackett gives away a free kick for an unfair challenge on Marlon Pack. Indirect free kick taken by Steve Elliott.
The ball is swung over by Keith Lowe, Lee Collins manages to make a clearance.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.