Goals from James Hanson, Rory McArdle and Nahki Wells earned Bradford promotion to League One as the Cobblers were brushed aside in an entertaining clash in the capital.
Hanson opened the scoring for Bradford with a looping header in the 15th minute after a cross came in from the right from Garry Thompson.
McArdle doubled the Bantams' advantage four minutes later with a header at the near post from Nathan Doyle, before Wells added a third goal on 28 minutes.
Wells fired in at the back post after a header across goal from Thompson, but Hanson looked to be in an offside position during the build-up to the goal.
The Bantams almost scored a fourth soon after but Kyel Reid's header was tipped over the bar by Northampton goalkeeper Lee Nicholls.
Town boss Aidy Boothroyd brought on Joe Widdowson and Adebayo Akinfenwa early in the second half for Lee Collins and Clive Platt in a bid to shake the Cobblers into action, and Akinfenwa fired wide in the box soon after.
Boothroyd made a last throw of the dice on 70 minutes when he brought on Lewis Hornby for Ishmel Demontagnac, with midfielder Luke Guttridge handed some instructions on a piece of paper which he read before tucking them in his shorts.
But the Cobblers just couldn't break through Bradford's resolute defence and could not muster a single shot on target, while City made a happy return to Wembley after their 5-0 Capital One Cup final defeat to Swansea.