Loic Remy's first-half strike saw Marseille record a first Ligue 1 win in 14 games at Stade Velodrome.
Remy, a reported target for Tottenham, slotted the only goal of the game on the half-hour mark from Mathieu Valbuena's superb pass as OM picked up three points for the first time since January 29.
The hosts were dominant for large spells and full-back Cesar Azpilicueta had an early chance for them but he was foiled by a sharp stop from Nancy keeper Guy N'Dy.
With Marseille having almost 70 per cent of possession chances kept coming their way, with Valbuena and Andre-Pierre Gignac bringing saves from N'Dy after being set up by Remy.
But the striker showed his team-mates how it should be done in the 30th minute, as he fired home left-footed from Valbuena's exquisite pass.
N'Dy saved Benoit Cheyrou's strike from the edge of the box early in the second half as OM continued to threaten.
Having spent the evening almost exclusively on the back foot Nancy almost grabbed an equaliser six minutes after the break as Djamel Bakar tried his luck from distance, but Steve Mandanda was equal to the task.
But normal service was soon resumed as Valbuena's flick played in Jordan Ayew, but his curling effort was denied by the on-form N'Dy.
Marseille were almost punished for their profligacy in front of goal in the dying moments but Mandanda made a superb agile save to keep out Salif Sane's fierce drive and Sebastien Puygrenier could not put home the rebound.