Dempster's first position as a doctor was a regent, or full professor, of the Collège de Navarre, at age 17. He soon left Paris for Toulouse, which in turn he was forced to leave by the hostility of the city authorities, aroused by his violent assertion of university rights. He was than elected professor of eloquence at the academy of Nîmes. A murderous attack upon him by one of the defeated candidates and his supporters was followed by a libel case, which, though he ultimately won, forced him to leave the town.