Mansfield, who had lost their last two fixtures, may feel that they could have the won the game after they controlled it for long periods.
They had the better chances in the opening moments of the game, with Rakish Bingham looking lively.
His meandering run in the ninth minute looked promising, before he saw his tame effort fall gratefully into the hands of Paul Jones.
Pompey were beginning to let Mansfield into the game and Fergus Bell was only too pleased to respond with a hopeful looking shot curling just wide a minute later.
The Stags continued to dominate without creating any significant chances, while Portsmouth were looking to counter and were reduced to shots from distance.
Mansfield finally turned their dominance into a goal just after half-time when Simon Heslop found himself free, sending a low shot right into the bottom corner.
It was an inspired swap too as Atangana made an immediate impact, bursting into the box and cutting back for Paul Robinson to slot home Portsmouth's equaliser as the game finished 1-1.