Mariam Soulakiotis was born around 1883 in Keratea, Greece, and was originally named Marina Soulakiotou. Before becoming known for her crimes, Mariam worked in a factory. Later, she became a nun in the Greek Orthodox Church and closely followed Bishop Matthew Karpathakis. Together, in 1927, they founded the Panagia Pefkovounogiatrissa Monastery near Keratea. Mariam became the abbess (leader) of this monastery.

Between 1939 and 1950, Mariam committed many serious crimes inside her monastery. She tricked wealthy women into joining the monastery and forced them to give their money and property to her. After getting their wealth, some women were reportedly killed or left to die from poor treatment.

The authorities started investigating when family members of victims complained. A Greek-American woman named Simela Spyrides disappeared at the monastery in 1949, causing more suspicion. Investigators also found that at least 150 children seeking medical help for tuberculosis died due to neglect under Mariam's care.

On December 4, 1950, police raided the monastery. They found many children and elderly women living in terrible conditions. Records showed around 500 people had given all their properties to Mariam before mysteriously dying in the monastery.

Mariam was arrested on December 5, 1950. She faced several charges, including murder, fraud, blackmail, and torture. Between 1951 and 1953, Mariam had three different trials and was sentenced to 14 years in prison. She died in prison (Averoff Prison) on November 23, 1954, at the age of 71.

Although some followers continued to see her as holy, most viewed Mariam Soulakiotis as a criminal responsible for many deaths and acts of cruelty.