Marc Richards put the Cobblers ahead in first-half stoppage time but Leon Clarke levelled after 61 minutes and John Fleck hit a winner to see Chris Wilder's men return to the second tier.
The Blades' result though was academic after Fleetwood, the only side that could deny them automatic promotion, went down to a 2-0 defeat to Oldham.
Nathan Pond put the ball into his own net in the 25th minute to edge the Latics into the lead and Lee Erwin's strike 10 minutes into the second half wrapped up victory.
Bolton confirmed at least a play-off place despite David Mirfin's 11th-minute strike for Scunthorpe giving them a 1-0 win over the Trotters.
But Southend's 2-1 defeat at Charlton meant they could no longer overhaul Bolton.
Ricky Holmes gave Charlton a sixth-minute lead but their advantage was short-lived as John White equalised five minutes later.
Substitute Jake Forster-Caskey though struck a 74th-minute winner for the Addicks.
Coventry kept their slim survival hopes alive with a 1-0 win over Peterborough at the Ricoh Arena.
Ruben Lameiras found the net six minutes into the second-period after Peterborough's Craig Mackail-Smith had seen his first-half penalty saved by Lee Burge.
Chesterfield saw their own hopes of retaining their League One status receive a boost after Joe Rowley's strike saw them beat Port Vale 1-0 for a first win in seven.
Jonathan Obika hit a last-minute equaliser to earn relegation-threatened Swindon a 1-1 draw with Milton Keynes Dons following Kieran Agard's penalty two minutes before the break at the County Ground
Stefan Payne got the only goal as fellow strugglers Shrewsbury earned a crucial 1-0 win over Rochdale and Steve Morison's goal earned Millwall a 1-1 draw with Gillingham after Joe Quigley had put the Gills ahead moments earlier.
Mark Marshall and Charlie Wyke scored as Bradford consolidated their promotion hopes with a 2-0 win over Bury and Billy Bodin's first-minute effort gave Bristol Rovers a 1-0 win at AFC Wimbledon.
Andreas Makris's first goal for Walsall gave them the lead against Oxford but Chris Maguire earned a 1-1 draw.
Source : PA
Source: PA