Montpellier, who surprisingly won their first league title against all the odds last season, struck the first blow when Senegalese forward Souleymane Camara turned John Utaka's pass into the net after a quick break in the 34th minute.
Toulouse, eighth last season, made their numerical advantage count when Wissam Ben Yedder headed in Serge Aurier's cross in the 72nd minute.
But they immediately pressed the self-destruct button when two of their players were dismissed in the next 12 minutes.
Paris St Germain, who have spent more than 100 million euros on their team, are the clear favourites to win the title. Montpellier have said they cannot compete with the Qatari-backed Parisians.