The Cobblers had to beat the Magpies and hope Accrington lost to Cambridge and Bristol Rovers failed to beat Crawley, none of which happened.
The Cobblers, now 14 points clear of second-placed Oxford, went in front through Ricky Holmes' sweetly-struck volley into the bottom corner.
Their joy was short-lived, however, as Notts County levelled four minutes later, Thierry Audel rising highest to head in a corner.
AFC Wimbledon beat Plymouth to secure League One promotion
Jon Stead put Notts County ahead from the penalty spot after he had been fouled, but Northampton equalised from their own spot-kick as Holmes converted.
Bristol Rovers moved back into the automatic promotion places as they eased to a 3-0 win against Crawley - their sixth straight home league victory.
They took the lead through Matty Taylor's close-range header from a corner - his ninth goal in eight games.
Liam Lawrence doubled the Pirates' advantage with a superb free-kick and Taylor took his tally for the season to 25 with his second of the game to make it three shortly afterwards.
Plymouth are fourth after Ollie Watkins' stunning late strike snatched Exeter a 2-1 win in the Devon derby.
Argyle had taken the lead through Jamille Matt's third goal in as many matches but Watkins levelled for Exeter with 10 minutes remaining and then scored his sixth goal in seven games to snatch victory in time added on with a superb solo effort.
Accrington drop to fifth but have the same number of points as Rovers and Plymouth after they were held to a 1-1 draw at home by Cambridge.
The U's broke the deadlock through Jimmy Spencer's fortuitous effort but Tariq Fosu pulled Stanley level.
Portsmouth are still firmly in the promotion picture after Michael Smith's strike earned them a 1-0 win against Carlisle.
There is now a five-point gap between Pompey in sixth and seventh-placed Wycombe, who were beaten 2-1 by AFC Wimbledon.
Lyle Taylor opened the scoring for the Dons with a simple finish but Wycombe levelled when Michael Harriman finished smartly into the corner.
Taylor made it 17 for the season from the penalty spot to win it for AFC Wimbledon.
At the bottom, Stevenage eased their relegation worries with a 1-0 win at Stevenage.
A Michael Tonge penalty five minutes from time saw them move nine points ahead of second-bottom York, who drew 1-1 at home to Leyton Orient.
Bradley Fewster opened the scoring for the Minstermen, tapping home from a yard out.
York defender William Boyle, who scored an own goal last weekend, was sent off for a second bookable offence in the 75th minute and seconds later it was 1-1 as Lloyd James fired in.
Elsewhere, two goals in the first eight minutes from Shaun Miller helped Morecambe to a 4-2 win against Barnet.
The Shrimps added a third when Alex Kenyon nodded in a corner but Michael Gash pulled one back for Barnet.
Cole Stockton grabbed a fourth for Morecambe in the 89th minute but there was still time for Gash to grab his second.
Hartlepool's resurgence continued as two Billy Paynter penalties earned them a 2-1 win against 10-man Mansfield and extended their unbeaten run to seven matches.
Krystian Pearce was sent off conceding the first spot-kick but they were unlucky not to take something from the game after Colin Daniel had equalised.
Jack Compton's early penalty secured Yeovil a 1-0 win against Newport.
Penalty levels League Two play-off first leg between Portsmouth and Plymouth Argyle
Oxford and Bristol Rovers grab promotion as Accrington Stanley blow it