The win lifted the 2010 French champions to provisional fifth place in the standings with 31 points from 19 games, six point off the pace, before a three-week winter break.
Nancy stayed in 15th spot on 18 points, one point above the safety zone with the rest of the mid-week Ligue 1 programme scheduled for Wednesday.
Leaders Montpellier travel to Evian Thonon Gaillard while second-placed Paris Saint-Germain, who trail only on goal difference, visit in-form Saint-Etienne and champions Lille, who are third, host Nice.
At Nancy, Marseille, who have won three of their last four league matches, were clinical in attack and scored from their first two clear chances.
France forward Valbuena, who helped OM beat Lorient last weekend in stoppage-time and put the former European champions into the Champions League last 16 with a last-gap goal at Borussia Dortmund, was once again on the spot.
After a swift one-two with Loic Remy, he raced into the box and opened the scoring with a low shot in the 18th minute.
Nancy's Lemaitre equalised from a Jean Calve cross in the 37th but Marseille's Cameroon midfielder Mbia headed home from a Morgan Amalfitano service four minutes before the break to give the visitors a deserved half-time lead.
The second period was much less lively, although substitute Gonzalez added a third for the visitors a minute into added-time to put the result beyond doubt.
