The American midfielder scored the only goal of the game on 16 minutes to condemn the hosts at the Stade Velodrome to consecutive defeats against other sides in the division's top six.
The French international stopper was at full stretch to prevent the hosts falling further behind, when he stuck out a leg to keep put Papy Mison Djilobodji's flick on from a Nantes corner on 26 minutes.
Marseille were tentative going forward and struggled to create open-play chances, with Florian Thauvin's 30-yard free kick, which landed on top of the net, the closest that they came in the first half.
The reported Arsenal target was his side's biggest threat after the interval too, but he side-footed wide from a corner, a few minutes after Nantes's Serge Gakpe curled over the crossbar from close range.
Elie Baup's outfit should have grabbed an equaliser when Saber Khalifa somehow scooped the ball over from just a few yards out, after he had been found unmarked at the back post by Andre-Pierre Gignac.