Goals from Andy Holt and Jason Crowe outgunned a spectacular 35-yard effort from Andy Williams and the Cobblers need just a point from their final two games to ensure they will not be making the drop.
Hereford created the lion's share of the chances in the first half, but fell behind in the 29th minute when a cross by Mark Hughes bypassed Adebayo Akinfenwa and Crowe, but Holt had time to control the ball and look up before smashing it past Peter Gulacsi in the visitors' goal.
Hereford could point at a glaring miss by top scorer Steve Guinan in the 25th minute when he skied the ball over the bar after Marc Pugh cross, but Guinan was desperately unlucky eight minutes before the break when he saw his fierce volley come back into play off the angle of post and crossbar.
Northampton goalkeeper Chris Dunn earned plenty of plaudits, tipping over a header from Richard Rose and denying Williams before the break but he was beaten six minutes after the restart by a spectacular long-range effort.
That set the home nerves jangling and Pugh almost gave the visitors the lead shortly afterwards, with a low drive which went flashing wide.
But after Akinfenwa had volleyed wide for Northampton, the home side regained the lead after 69 minutes when the striker laid a path into the path of Crowe on the edge of the box and the midfielder came steaming in to fire a drive past Gulacsi.
That was Crowe's eighth goal of the season and he almost made it safe nine minutes later only to fire wide.
That meant the home nerves were on edge until the bitter end with Hereford's best chance a header by Guinan from a Williams cross which fell straight into the grateful arms of Dunn.