Petiot was born in 1897 at Auxerre, France. A psychiatrist diagnosed him as mentally ill in 1914, and he was expelled from school many times.
Petiot's first victim might have been Louise Delaveau, with whom he had an affair in 1926. Delaveau disappeared in May and neighbours later said that they had seen Petiot load a trunk into his car. Police investigated, but eventually dismissed her as a runaway. That same year, Petiot ran for mayor of the town - and won. Once in office, he embezzled from the town funds. In 1927 he married Georgette Lablais. Their son Gerhardt was born the next year.
The local prefect received numerous complaints about Petiot's theft and shady financial deals and Marcel Petiot was eventually suspended as a mayor in 1931, and resigned. The village council also resigned in sympathy. Five weeks later, on October 18, he was elected as a councilor for the Yonne district. In 1932 he was accused of stealing electric power from the village of Villeneuve-sur-Yonne and he lost his seat in a council. Meantime, he had already moved to Paris.
In Paris Petiot attracted patients with his imaginary credentials and built an impressive reputation for his practice at 66 Rue Caumartin. However, there were rumors of illegal abortions and overt prescriptions of addictive remedies. In 1936 he was appointed médecin d'état-civil with authority to write death certificates.
Petiot also claimed that he had developed secret weapons that killed Germans without leaving forensic evidence, had high-level meetings with Allied commanders; engaged in resistance activities, and worked with a group of anti-fascist Spaniards.
Petiot's most lucrative activity, however, was his own false escape route, Fly-Tox. He adopted a "code-name" "Dr. Eugène." He accepted anyone who could afford his price of 25,000 Francs per person regardless of whether they were Jews, resistance fighters, or ordinary criminals. Petiot claimed that he could arrange a safe passage to Argentina, or elsewhere in South America. He also claimed that Argentinean officials demanded inoculations and injected his victims with cyanide. Then he took all their valuables and disposed of the bodies. People who trusted him to deliver them to safety were never seen alive again.
At first Petiot dumped the bodies in the Seine River, but he later destroyed the bodies by submerging them in quicklime or by incinerating them.
The Gestapo eventually found out about him and by 1943 they had heard all about his "route." Gestapo agent Robert Jodkum forced prisoner Yvan Dreyfus to approach the supposed network, but he simply vanished. A later informer infiltrated the operation and the Gestapo arrested part of the network - under torture they confessed that "Dr Eugene" was Marcel Petiot.
On March 6 1944, neighbours noticed that the smoke from the chimney of 21 Rue le Sueur in Paris smelled noxious. When they went to complain on March 11, they found a note on the door that said the resident would be away for a month.
Neighbours notified the police and told them that Petiot owned the house. When police called Petiot, he told them to wait for him. However, 30 minutes later, police were obliged to call the fire department to stop the spreading fire. When firemen came through a second-story window, they found a grisly display of bodies and body parts.
When Petiot arrived, he claimed that he was a member of the French Resistance and claimed that the bodies were those of Germans, traitors, and collaborators. Because people in general approved of resistance activities, the police were reluctant to arrest Petiot, and so they released him. When police searched the garage, they found a pit filled with quicklime with human remains in it. On the staircase they found a canvas sack containing human remains. There were enough body parts for at least ten complete bodies.
During the following months Petiot hid with friends, claiming that the Gestapo wanted him because he had killed Germans and informers. When the Resistance and the Paris police rose against German troops in Paris, Petiot adopted the name "Henri Valeri" and joined the French Forces of the Interior (FFI). He became a captain in charge of counterespionage and prisoner interrogations. Finally, on October 31, Petiot was recognized at a Paris metro station, and arrested.
Petiot was placed on death row at La Santé prison. He continued to claim that he was innocent and that he had only killed enemies of France. He claimed that he had discovered the pile of bodies in 21 Rue le Sueur in February 1944, and assumed that they were collaborators that members of his "network" had killed.
Prosecutors eventually charged him with at least 27 murders for profit. Their estimate of his loot ran to 200 million francs.
Petiot went on trial on March 19, 1946, facing 135 criminal charges. His defence attempted to portray Petiot as a resistance hero, but the judges and jurors were unimpressed. Petiot was convicted of 26 counts of murder, and sentenced to death.
On May 25, Petiot was beheaded.