Thénardier runs an inn in Montfermeil with his wife, exploiting Fantine's desperation by taking young Cosette as a ward while demanding ever-increasing payments for her care. He and his wife subject Cosette to severe neglect and abuse, forcing her into hard labor while favoring their own daughters, Éponine and Azelma. The family profits through deceit, overcharging guests and inventing false expenses.

After financial ruin costs them their inn, the Thénardiers move to Paris, adopting the alias "Jondrette" at the Gorbeau House. They live in extreme poverty and coordinate criminal schemes, including writing fraudulent letters to solicit money. Thénardier partners with the Patron-Minette gang to conduct robberies. In February 1832, he orchestrates a trap for Jean Valjean, luring him under false pretenses of charity. Recognizing Valjean and Cosette, Thénardier and the gang ambush Valjean upon his return, intending robbery and violence. Inspector Javert arrests them during the confrontation, imprisoning Thénardier, his wife, and Azelma.

While imprisoned, Thénardier engineers an escape with the Patron-Minette. He later attempts to extort Marius Pontmercy by falsely accusing Valjean of crimes. Deviating from the novel, Thénardier escalates this scheme into a plot to murder Marius. Javert intervenes during the attempt, rearresting Thénardier. He remains incarcerated with no sign of reform or redemption.

His character displays consistent greed, manipulation, and physical cowardice. He shows no genuine affection for his family, viewing them as tools for his schemes. His relationship with his wife is fractious, marked by mutual disrespect. Unlike his wife and younger daughter, who gain release and return to restart the Montfermeil inn without him, Thénardier remains defined by criminal pursuits, ending the story imprisoned, reflecting an unchangeable nature.