Oxford lost 3-2 at home to Luton while Accrington Stanley drew 2-2 with Morecambe, meaning that nobody can now catch Northampton at the top of the table.
Bristol Rovers moved into third with a 2-1 win over Yeovil and goals from Rory Gaffney and Matt Taylor.
Plymouth continued their surge towards an automatic promotion spot by beating Portsmouth 2-1 thanks to goals from Jamille Matt and Gregg Wilde after Michael Smith's opener.
AFC Wimbledon beat Plymouth to secure League One promotion
Paul Robinson's 83rd-minute goal against Crawley Town keeps AFC Wimbledon in the hunt for a play-off berth.
At the other end of the table, Dagenham and Redbridge were relegated out of the Football League as they lost 3-2 to managerless Leyton Orient.
York City look likely to join them after a 2-1 defeat at the hands of Hartlepool, but Stevenage are safe now after beating Newport County 2-1.
The biggest win of the day went to midtable Mansfield, who trounced Notts County with goals from Adam Chapman, Reggie Lambe, Mitchell Rose and a brace from Matt Green.
Cambridge versus Carlisle was goalless, while Barnet came from behind to secure a 1-1 draw at Wycombe.
Penalty levels League Two play-off first leg between Portsmouth and Plymouth Argyle
Oxford and Bristol Rovers grab promotion as Accrington Stanley blow it