Brave injury-ravaged Orient snatched a last-gasp equaliser three minutes into injury time as Mansfield conceded a vital late goal yet again.
Just 60 seconds after Stags striker Iyseden Christie headed Neil MacKenzie's curling cross against the bar Orient striker Gary Alexander nodded in substitute Lee Thorpe's deep centre, three days after the home side had crashed out of the Carling Cup to Sunderland's injury-time winner.
However the equaliser was nothing more than Orient deserved for a courageous fightback after falling behind to a controversial goal.
The O's dominated the second half. David Hunt lashed a 15 yard drive against the post Thorpe's shot was well saved by Kevin Pilkington and Tom Newey's fierce finish was acrobatically kept out by the busy keeper.
But then Alexander, denied a first-half hat-trick by Pilkington's heroics, had the last laugh.
Earlier the Stags should have got off to the perfect start. In the fifth minute Christie's flick-on sprung the offside trap, only for the unmarked Andy White to scuff his finish and allow keeper Glenn Morris to make a vital save.
Sixty seconds later Christie, playing against his former club, was unmarked inside the area, but Matthew Joseph slid in to block his shot.
In the 13th minute Mansfield took a controversial lead. Reserve keeper Morris, replacing the injured Lee Harrison, spilled Wayne Corden's free-kick under pressure from Dave Artell and with the visitors appealing for a free-kick Luke Dimech tapped into an empty net.
Morris redeemed himself with a fingertip save to push Christie's 25-yard rising drive over the bar.
But from then on Orient recovered and dominated.
Before half time Pilkington saved on the line on three occasions from Alexander's quick shots on the turn and the keeper also did well to hold on to Justin Miller's long-range effort.