When head prosecutor in Valenciennes, he became famous for prosecuting businessman-cum-politician Bernard Tapie for rigging the match between his soccer club (Olympique de Marseilles) and the local club of Valenciennes. Tapie was sentenced to prison in 1995 for complicity of corruption and subornation of witnesses.
In 2003, as head prosecutor of Nice, he returned to the limelight with allegations that the local judiciary deliberately derails Justice by miscarrying procedures, letting prescription lapse, or losing files, in "sensitive" cases; he also indicated that some members of the judiciary had unhealthy contacts in Masonic lodges with local personalities, including those they have to prosecute. He also opened a criminal investigation targeting rock star Johnny Halliday for allegedly raping an employee of a yacht rented by Halliday. He had also hinted at possible investigations concerning corruption in the city of Nice's administration. An official report from the inspection corps of the Justice ministry blamed him for unwarranted accusations against his colleagues, but supporters of Montgolfier argue that he is criticized merely because he has uncovered cases involving well-connected people.