Defeats to PSG, Rennes and Lyon in their last five matches put Marseille's position at the top of the table under threat, but they were given few problems by in-form Nantes at Stade Velodrome.
Florian Thauvin broke the deadlock after 24 minutes, as the winger raced in front of his marker at the near post and directed Dimitri Payet's cross in with a delightful flick of his left foot.
Nantes, who went into the match on the back of a 10-man unbeaten run in all competitions, should have drawn level in the 36th minute, but Steve Mandanda made a fine save from George N'Koudou.
And Marseille capitalised on that let-off just three minutes later, as Rod Fanni guided Thauvin's delivery beyond the helpless Remy Riou with the back of his head.
Fourth-placed Nantes improved in the second half, but Marseille held on with relative comfort ahead of PSG's clash with Nice on Saturday.
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