Oli Hawkins scored the decisive goal midway through the first half as Mansfield created a new club record of 11 successive home wins.
It breaks a record which had stood since 1949.
Stags were dominant throughout and outclassed Cobblers for large parts of the game, with only the heroics of keeper Liam Roberts denying them a more comfortable win.
They were helped on the way after Paul Lewis was dismissed for a petulant shove off the ball with 15 minutes to go.
But, in truth, there was only likely to be one outcome after Mansfield showed their class throughout.
It leaves Stags outside the automatic promotion zone on goal difference ahead of Tuesday’s trip to leaders Forest Green Rovers.
James Perch missed out through illness, Jamie Murphy missed out due to a hamstring injury, while Jordan Bowery returned to the bench.
The visitors saw the first sight of goal with Fraser Horsfall having a soft header saved by Nathan Bishop inside the first 30 seconds.
Stephen McLaughlin was booked on five minutes for a crunching, but what seemed a fair tackle, on Aaron McGowan.
Chanka Zimba failed to trouble Bishop after a ball over the top caught Mansfield out.
Oli Hawkins headed Mansfield in front from Matty Longstaff’s deep cross on 21 minutes.
It heralded a brief spell of pressure for Stags with Northampton forced to defend a number of dangerous crosses.
Jack Sowerby smashed a shot well wide after a corner fell perfectly, before Mitch Pinnock hit the crossbar with a curling effort.
McLaughlin fired over from distance on 40 minutes as Mansfiield hit on the counter-attack.
Liam Roberts made a brilliant one-handed diving save to keep out Longstaff’s curling shot a minute before the break.
The Cobblers keeper did well to get down low to his near post to save from Rhys Oates seconds later.
And he was at it again when he stuck out a leg to keep out another powerful shot from Oates in stoppage time.
Lucas Akins fired just wide and Stephen Quinn had a shot charged down as Stags started the second-half strongly.
Quinn had a shot deflected just wide after a good break on 65 minutes as Mansfield pushed for the decisive second.
Roberts got down well to keep out an Akins shot across the face of goal, before Stirk had a shot deflected wide in the last minute.
Mansfield Town: Bishop, Hewitt, O'Toole, Hawkins, McLaughlin, Longstaff (Wallace 90+2), Stirk, Quinn (Rawson 90+4), Lapslie, Oates (Bowery 72) , Akins.
Subs not used: Maris, Johnson, Stech, Law.
Northampton Town: Roberts, McGowan, Horsfall, Guthrie, Mills (Rose 81), Sowerby, Hoskins, Lewis, Pinnock, Lubala (Eppiah 60), Zimba (Appere 71).
Subs not used: Harriman, Pollock, Dyche.
Referee: Anthony Backhouse.
Chad man of the match: Stephen Quinn.
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