Serge Gakpe's long-range effort crept under the Marseille goalkeeper to rescue a deserved point for Friday's hosts at the Stade de la Beaujoire.
The visiting captain's ruck will have been doubly galling for team-mate Florian Thauvin, who had given Marseille the lead with a stunning first-half effort.
A point leaves Marseille four points off fourth-placed Saint-Etienne, whom they must overtake to secure a UEFA Europa League place for next season, with just three games left to play.
Nantes appeared to be getting on top in the early stages when Thauvin gave the visitors the lead in fine fashion.
Andre-Pierre Gignac's neat flick sent Thauvin away and, from 20 yards, the playmaker unleashed a stunning left-foot shot into the corner.
Banel Nicolita and Alejandro Bedoya passed up good opportunities to bring the hosts level before the break, and they returned strongly for the second period, but Jordan Veretout fired an effort off-target.
Gignac flashed a dangerous ball across the face of goal as Marseille sought to move further ahead, but Mandanda's error brought Nantes level.
Gakpe's effort from distance was firmly hit, though the visitors' captain should have done much better as he failed to get down quick enough.
