Chair caught Stags out pushing for a winner to seal a smash and grab win for a Stevenage side still in with outside hopes of the play-offs.
With rivals MK Dons losing at Colchester, Stags could have made next weekend’s last game at MK a little easier for themselves.
But they found themselves behind to a goal against the run of play by Kurtis Guthrie on 71 minutes.
Tyler Walker’s 26th goal of the season revived home hopes on 84 minutes, but they were ultimately punished for going for broke for a winner at the death.
Mansfield made one change, which was in midfield where Will Tomlinson took over from Jorge Grant, who dropped to the bench.
Stags began the day in the top three only on goal difference ahead of MK Dons, who play at Colchester.
Stags’ other promotion rivals Bury had their trip to Tranmere postponed until Tuesday due to a waterlogged pitch to prolong the tension.
The game kicked off in a tremendous atmosphere and referee Drysdale produced a yellow card after only three minutes after Timlin hacked down Tomlinson.
But the Stags slowly began to build up a head of steam and push the visitors back into their own box.
Keeper Farman had his first save to pull off on eight minutes as Benning flashed a free kick towards the near post from 25 yards out on the right and Farman dived to his left to turn it round his post.
On 15 minutes a lovely backheel from Mellis put Tomlinson into space for a low shot that Farman again had to push round at full length.
Walker then sent Hamilton on his way on the right, but his shot was well over.
But there was danger on 22 minutes as Stags failed to clear a free kick from the left and Nugent had clear sight of goal only to see Logan superbly beat away his finish from 10 yards.
A minute later alarm bells were ringing as Guthrie’s flicked header took a deflection onto the post and out for a corner.
Suddenly the home tension was almost unbearable as Stevenage racked up a string of corners.
On 31 minutes Guthrie burst past three opponents into the box and went down under a challenge, but Drysdale said no penalty.
A minute later Logan made another save as he clawed round a long range effort from Chair, and soon after Sonupe drilled a low shot just wide as the home fans turned up the noise to try to lift their players.
Stags were angry on 41 minutes as Nugent tripped Hamilton as he tried to burst clear into the box and no card was shown.
But a tense half ended goalless.
A minute into the restart Tomlinson was booked for his foul on Sonupe.
Farman had to push a Benning corner away as it threatened to curl straight in as Stags began to press hard, winning four early corners.
On 58 minutes the visitors got upfield for the first time and Sonupe was wide with a shot on the turn.
Seconds later Walker was wide from 20 yards as the ball sat up invitingly for him to prod it goalwards.
Still desperately needing a goal, Stags sent on striker Rose in place of midfielder Tomlinson with 25 minutes to go.
Rose soon had a chance but his header sailed over from Benning’s cross.
Instead it was the visitors who took a shock lead against the run of play as Guthrie controlled a cross, turned and smashed a low volley past Logan.
Stags quickly sent on Grant for Jones as the pressure mounted.
On 79 minutes Mellis had a shot parried by Farman and Hamilton saw his follow-up superbly blocked by Nugent.
But on 84 minutes Stags were level. Logan’s kick caught on the wind and Walker nipped through the defence to beat Farman to it and guide it into the empty net.
In a last throw of the dice, MacDonald replaced Benning on 90 minutes.
But, as they pushed forward, Stags were caught out in the third added minute of five as Chair fired a low shot inside the left post from the edge of the box to break home hearts.
MANSFIELD: Logan; Sweeney, Peace, Preston; Jones (Grant 73), Bishop, Tomlinson (Rose 65), Mellis, Benning (MacDonald 90); Walker, Hamilton. Subs: Smith, Atkinson, Turner, Hakeem.
STEVENAGE: Farman, Wildin, Timlin, Cuthbert, Byrom, Sonupe (McKee 81), Chair (Wilkinson 90+4), Vancooten, Guthrie, Nugent, Martin. Subs: Byrne, Hunt, Revell, Gibson, Smyth.
REFEREE: Darren Drysdale of Lincolnshire.
ATTENDANCE: 6,390 (209 away).
CHAD STAGS MAN OF THE MATCH: Krystian Pearce.