It took a 90th minute header from Junior Agogo to deny Colin Calderwood's former side victory, after Forest had only a Lewis McGugan strike in the 15th minute to show after relentless first-half pressure.
Northampton took full advantage as a mistake from Matt Lockwood gave Poul Hubertz the chance to level in the 58th minute, before full-back Daniel Jones hit a spectacular second seven minutes.
But the Cobblers were denied a valuable win, as Agogo spared Forest's blushes by heading home a McGugan corner.
It was Forest who dominated the early possession, but without creating any clear-cut opportunities.
While the visitors twice offered a warning that they were a threat when they broke quickly, as firstly Bradley Johnson and then Ryan Gilligan let fly with long-range efforts, although neither effort troubled Paul Smith's goal.
Forest's build-up remained patient and they could have had a penalty when Mark Hughes appeared to handle a flicked shot from Nathan Tyson, with referee Penton ruling that it had struck the defender's chest.
The Reds came even closer as a neat Kris Commons turn on the edge of the box was followed by a spectacular shot that rattled the bar, with Tyson failing to control his shot on the rebound.
And the goal was not long in coming, as a fantastic 50-yard pass from James Perch sent Commons into space on the right, where the winger cut the ball back perfectly for McGugan to drive a powerful, rising shot inside the post from just inside the box.
Forest's confidence was high and they almost had a second as Chris Cohen sent Tyson charging into the box. His shot was blocked but Commons came close as he sent the loose ball dipping narrowly over the bar.
Hubertz almost deflected the ball into his own net as he sent a headed clearance from a corner flashing wide of the post.
While Perch came equally close as he connected with a Cohen cross but saw his header loop agonisingly wide.
Northampton were hanging on, but Forest were also guilty of missing chances as Tyson twice failed to trouble the keeper after being sent clear of the defence.
Ian Breckin came close with an early second-half header which flew inches wide, while Northampton's Liam Dolman, Gilligan and Mark Hughes were both booked for late tackles.
Smith had to produce an outstanding save to deny Northampton an equaliser, as Hubertz sent a left-foot shot bending towards the bottom corner.
But Forest gifted their visitors an equaliser when Lockwood gave away possession at left-back and allowed Gilligan to send a cross into the middle, where Hubertz provided a simple close-range finish.
And it got worse seven minutes later, as Jones lashed the visitors into the lead with a spectacular 25-yard drive that beat Smith at his near post.
While Northampton, after surviving a first-half onslaught, could have had a third as Smith produced a spectacular save to deny Jason Crowe when he attempted a 35-yard lob.
But Agogo arrived to head Forest a precious equaliser in the 90th minute.