He was one of the leaders during the Reign of Terror. Believing Mr. Peabody to be one of the aristocrats oppressing the common folk of France, he tries to have him guillotined. Mr. Peabody managed to escape, and he and Sherman flee into the sewers. Robespierre catches up with them, and engages in a sword fight with Peabody, which Peabody wins. Mr. Peabody and Sherman then escape by flooding the sewer.
Later on in the film, when a paradox causes people and objects from the past to be pulled to the present, Robespierre is among them, crashing onto the WABAC's windscreen. When a police officer tells him to back away, he refuses, and is promptly tasered. Shortly after, the dazed Robespierre joins in on saying "I'm a dog too", adding that he is a poodle. When the time rift is repaired, he returns to his own time and tries to tase his men when they don't cooperate, but he ends up tasing himself.