The Stade Louis II faithful were expecting a victory, after Marseille made five changes from the team that beat Newcastle United in the Uefa Cup semi-finals, which included resting the brilliant Didier Drogba.
Monaco named a near full strength team with only Patrice Evra and Edouard Cisse not in the team which beat Chelsea in midweek.
The game was a close affair, with l'OM having the better of the opening exchanges.
Monaco came back into the game with Ferando Morientes leading the line well - but there was no breakthrough.
The Spaniard was withdrawn with 20 minutes left - just as Drogba was introduced by the visitors.
Just as the game looked to be heading for a goalless draw, Monaco's playmaker supreme Ludovic Giuly (pictured) grabbed the winner with just a minute left.
The French star - who had hit the post just seconds earlier - struck from close range after Fabien Barthez parried a Lucas Bernardi drive into his path..
Monaco are still very much in the hunt for the title following the win as they are just two points adrift of leaders Lyon - with three games remaining.
Marseille are still adrift in seventh and realistically there only way into Europe next season will be by winning the Uefa Cup.
