The visitors had lost John-Joe O’Tooole for flooring Ritchie Sutton off the ball on 36 minutes in what was an ill-tempered first half.
Vadaine Oliver had then put home noses in front on 48 minutes.
But nervy Mansfield failed to capitalise on that situation as the visitors piled on the pressure, despite their numerical disadvantage, with the bar and Lenny Pidgeley denying them.
The game then had a nasty late twist just when it seemed Stags had clinched three vital points as referee Lee Collins decided Junior Brown had brought down sub Ivan Toney and pointed to the spot.
Marc Richards was cool as a cucumber as he lashed the spot kick down the middle to spark noisy celebrations for their 1,210 army of followers and the draw felt like a defeat for the home side.
With injury problems mounting, boss Adam Murray had to name himself on the bench.
He also took the gamble of pushing Ricky Ravenhill back into the side early after he hurt his hamstring in the warm-up last weekend.
However, Leicester loanee Callum Elder missed out with a calf injury so youngster Jack Thomas was given a full debut as one of four changes and had an immaculate afternoon.
Injured defender Michael Raynes was replaced by Martin Riley.
Out also went Matt Rhead, Jamie McGuire, both dropped to the bench. Recalled on the wing was Brown.
Northampton, playing towards their huge bank of travelling fans, began on the front foot and Horwood had a dangerous early low cross hacked away.
Then Ravenhill did well to hook away a cross by D’Ath from the other flank.
That conceded a corner which Oliver headed clear.
Stags finally got forward and Heslop tested Duke with a powerful low shot from the right of the box.
In attempting to get away a follow-up shot Brown felt his standing leg was taken in the box but no penalty was forthcoming.
Pidgeley had his first save to make on 13 minutes as Holmes advanced down the centre and let fly. The keeper blocked to his right and watch Holmes fire a follow-up into the sidenetting.
On 20 minutes Ravenhill was booked for bringing down Holmes.
Pidgeley then stretched up to safely clutch the free kick.
Riley headed away the Cobblers’ second corner on 24 minutes as the visitors continued to hold the edge.
Good hustling by Mansfield as the visitors tried to clear their lines saw the ball run loose and Heslop had a shot blocked to earn the Stags’ first corner.
When Brown got in his cross from the left by-line at the second attempt Heslop pumped in a shot that hit the ground first and bounced up at Duke who made a comfy safe.
Taylor was booked for bringing down Ravenhill on 29 minutes.
There were huge cheers for youngster Thomas as he slid in to win the ball and then won Stags a free kick just outside the Northampton box on the half-hour.
From 22 yards out Heslop then curled a superb free kick over the wall and saw it hit the top of the bar and go out for a goalkick.
Two minutes later a long Pidgeley clearance made its way through some poor attempts to defend it and rolled invitingly for Oliver to rifle a low shot across the face of goal and wide of the far post.
The game exploded on 36 minutes as Sutton was floored in the box by O’Toole. Pidgeley was livid and ran at the referee who had already decided on a straight red.
But tempers were boiling as Sutton stayed down and there was a lot of pushing and shoving.
The blood stayed pumping as Richards went in late on Ravenhill, then Sutton and Holmes clashed after another foul on the Stags man, the referee this time booking them both.
Duke came to confidently gather Stags’ second corner as half-time approached.
A minute into the three added Sutton’s square pass found Lambe on the edge of the box who delayed his shot and finally worked enough space to test Duke, who save low to his right.
The Cobblers won a corer in the last minute of the three, but it was over-hit and went straight out for a goalkick as a fractious 45 minutes ended scoreless.
However, Stags swept ahead within three minutes of the restart.
Beevers sent in a long free kick from wide on the right and poor defending allowed it bounce through to Oliver who guided a low finish past Duke at his near post from five yards for his sixth of the season.
The visitors almost had a quick reply as, on 51 minutes, Taylor headed against the home bar from D’Ath’s left wing corner – a real let-off for Murray’s men.
Horwood was lectured for shoving Thomas after the youngster had won another superb sliding tackle.
Thomas then won more applause for a fantastic run down the right which almost set up Lambe.
Cresswell got away with a hand across the face of Tafazolli as they challenged for a free kick.
Byrom’s follow through challenge after Riley had cleared the ball earned him a yellow card on the hour.
Horwood went down holding his face after Lambe had challenged him, though no contact seemed to be made that high. The referee decided to award a free kick only, which Pidgeley punched away.
Ravenhill headed away Northampton’s sixth corner before Thomas was shown a yellow card for a foul on 71 minutes.
Pidgeley superbly smothered a Horwood cross from the by-line as the visitors continued to press.
The Cobblers came close again as D’Ath cut inside Beevers to make space for a finish that was brilliantly blocked by Tafazolli. From the corner, Moloney pulled a low shot wide.
Stags made a change on 74 minutes as Bingham replaced Thomas.
Heslop saw the game’s eighth yellow card on 77 minutes for holding as the home side continued to toil
From a neatly-worked free kick Cresswell saw a six yard header strike team mate Richards in front of goal.
On 84 minutes Riley’s mistimed challenge saw him added to the growing list of names in the book as Northampton’s 10-men gallantly pressed hard.
With four minutes left the alert Pidgeley had to dive to his left to superbly turn away a 25-yard free kick from Horwood.
Five minutes were added on with Stags defending bravely.
But three minutes into that stoppage time, the visitors finally drew level.
Sub Toney turned inside Brown, who seemed to lean out towards him and bring him down.
Stags were livid with the referee, but his mind was made up.
Up stepped the cool head of Richards to drill home his spot kick down the centre and break home hearts.
STAGS: Pidgeley, Sutton, Riley, Tafazolli, Ravenhill, Heslop, Lambe (Murray 86), Beevers, Thomas, Brown, Oliver. Subs not used: Studer, Rhead, Bingham, McGuire, Marsden, Kee.
NORTHAMPTON: Duke, Moloney, Horwood, Collins (Toney 82), Richards, O’Toole, D’Ath, Taylor Bodin 90+1), Byrom, Holmes (Newey 90+4), Cresswell. Subs not used: Carter, Hackett, Tozer, Snedker.
REFEREE: Lee Collins of Surrey.
ATTENDANCE: 4,614 (1,210 away).
CHAD STAGS MAN OF THE MATCH: Jack Thomas.