Mary Flora Bell was born on May 26, 1957, in Corbridge, Northumberland, England. She was the second child of Elizabeth "Betty" Bell and William "Billy" Bell. Betty was a known local prostitute who often left her children in the care of their father, if he was at home. While Mary believed Billy Bell was her father, there are questions about whether he was her biological parent.

Mary had an unstable early life. According to her aunt, minutes after her birth, Betty expressed her dislike for the baby, shouting at hospital staff to take her away. Mary was unwanted and often neglected. She suffered injuries from accidents while in her mother's care. Betty once dropped Mary from a first-floor window and also gave her sleeping pills on multiple occasions. There was an incident where Betty sold Mary through an adoption agency to a mentally unstable woman. This led to her older sister and maternal aunt having to travel to reclaim her.

Despite the offers from family to take care of Mary, Betty refused to let anyone else look after her. Reports suggest that as a dominatrix, she may have even allowed some of her clients to abuse Mary.

Mary showed signs of behavioral issues from a young age. She had mood swings and would wet the bed. She often got into fights with other children. Some classmates described her as violent, recalling instances where she attempted to strangle them or even block their airways. This behavior led other children to avoid Mary. She did find some companionship with a neighbor's daughter named Norma Joyce Bell, who was not related to her.

On May 25, 1968, Mary Bell, who was just a few days shy of her 11th birthday, killed four-year-old Martin Brown in an empty house on St. Margaret's Road in Newcastle. Mary is believed to have acted alone during this crime. Three children later discovered Martin's body. He was lying on his back with no obvious signs of violence except for some blood and foam around his mouth. Despite attempts at resuscitation by a local workman, Martin was pronounced dead. The cause of his death was initially unclear, leading to an open verdict at the inquest.

On May 26, Mary and her friend Norma Bell broke into a nearby nursery. They vandalized the place, tearing up books and creating chaos before leaving four notes that seemed to reference Martin's murder. Authorities dismissed these notes as childish pranks.

On July 31, 1968, Mary and Norma were seen playing with three-year-old Brian Howe, who disappeared later that day. His body was found later that night in a nearby area known as "Tin Lizzie." He had been strangled. The coroner found clear signs of trauma, including bruises on his neck and puncture wounds on his legs. Broken scissors were also found at the scene, leading to more questions about what happened.

The investigation intensified after Brian's body was found. Over 1,200 local children were interviewed, including Mary and Norma. Although they played with Brian, the girls denied having seen him after lunch. However, during further questioning, Mary made statements that contained details only the police knew, which drew suspicion.

On August 4, Norma confessed to police that Mary had shown her Brian's body and admitted enjoying the strangulation. This statement led police to focus on Mary as the primary suspect. Forensic evidence linked Mary to the crime scene through fibers from her clothing found on Brian's body, connecting her to both murders.

Both girls were formally charged with Brian's murder on August 7. During the investigation, Mary also admitted to being present for Martin's death but claimed that Norma was responsible.

Following their arrest, Mary and Norma underwent psychological evaluations. Mary was identified as having a psychopathic personality disorder. The trial began in December 1968, where both girls pleaded not guilty to the charges. The prosecution argued that both acted together with the intention of causing harm.

After a brief deliberation, the jury found Mary guilty of manslaughter for both murders due to diminished responsibility, while Norma was acquitted. Mary was sentenced to be detained at Her Majesty’s pleasure, making her the youngest female killer in Britain at that time.

Mary was placed in a secure facility, where her behavior attracted notable attention. She later escaped from prison but was recaptured. In 1980, after spending over 11 years in custody, Mary was released with anonymity to help her start a new life. She later became a mother, but her past followed her, leading to different challenges in her life.