Léa Papin was born on September 15, 1911, in Le Mans, France. She was the daughter of Clémence Derré and Gustave Papin. Her early life was marked by family turmoil. When she was very young, she was given to her maternal uncle to raise. She lived with him until he passed away.

Léa had two older sisters, Émilia and Christine. The family's situation continued to worsen. Clémence sent Émilia to the Bon Pasteur Catholic Orphanage after a troubling incident with their father, Gustave. Clémence believed that Émilia had seduced him, resulting in her being sent away. Soon after, Christine and Léa also came to live at the orphanage. Clémence planned for them to stay there until they turned 15, at which point they could start working.

The mother, Clémence, and Gustave divorced in 1913. Émilia chose to enter a convent in 1918, effectively ending her relationship with her family. The sisters, Christine and Léa, worked hard in their roles at the orphanage. Christine trained in various household duties, hoping to become a nun, but that dream was not supported by their mother.

Christine and Léa were described in different ways. Christine was seen as a hard worker and a skilled cook, though she could be disobedient. Léa, on the other hand, was quiet, introverted, and obedient. People thought she was not as intelligent as her sister. The sisters worked as maids in different homes around Le Mans. They preferred to work together whenever possible, relying on each other as they managed the demands of their jobs.

On February 2, 1933, Christine and Léa Papin committed a brutal murder in the home of their employers, the Lancelin family. That evening, Monsieur Lancelin was out for dinner. Madame Léonie Lancelin and her daughter Genevieve returned home after shopping and noticed that the house was dark. The Papin sisters explained that a power outage had occurred due to a faulty iron that Christine had plugged in.

Madame Lancelin became angry and attacked the sisters. In the chaos, Christine attacked Genevieve, gouging her eyes out, while Léa did the same to Madame Lancelin upon Christine's order. Christine then ran to fetch a knife and a hammer from the kitchen, returning upstairs to continue the attack. The sisters used various weapons, including a heavy pewter pitcher, to bludgeon and stab the two women. Their rage led them to inflict severe mutilations on the bodies.

Later that evening, Monsieur Lancelin returned home and found the house dark. After discovering that his family was missing from the dinner party, he returned home suspicious. Unable to enter through the bolted front door, he and his son-in-law went to a police station for help. A policeman entered the home by climbing over the garden wall.

Inside, the officer found the gruesome scene of Madame Lancelin and Genevieve. Their bodies were barely recognizable, having been bludgeoned and stabbed multiple times. The policeman thought that the Papin sisters had also been harmed, but when he went upstairs, he found their bedroom locked. After gaining access with the help of a locksmith, he discovered Christine and Léa in bed together, with a bloody hammer nearby.

During questioning, both sisters confessed to the killings. They claimed that it was done in self-defense. Throughout the trial, they alternated responsibilities for the crime and generally supported each other. They were placed in prison and soon separated, causing significant distress for Christine. Meanwhile, their trial began, and their lawyer argued they were insane, but medical evaluations concluded otherwise. It was determined that they suffered from "Shared Paranoid Disorder."

After a brief deliberation, the jury found the Papin sisters guilty. Léa received a 10-year sentence, while Christine was initially sentenced to death, which was later changed to life in prison. After their imprisonment, Christine's mental health worsened, leading to her eventual death in 1937. Léa, however, served eight years before being released in 1941. She lived under a false name for many years until her death in 2001. The sisters were buried together in Nantes.