Mary Bell was born in May of 1957, when her unwed, mentally unstable mother was, herself, a child of seventeen. Though Betty Bell would subsequently wed the baby's father, marriage did not guarantee a stable home. Mary's father was frequently out of work, occasionally in trouble with the law. Betty, for her part, frequently left her daughter with relatives or acquaintances, once "giving" the child to a woman she met on the street, outside an abortion clinic. The Bell home, in Newcastle, England, was filthy and sparsely furnished. At school, Mary became known as a chronic liar and disruptive pupil. On occasion, she voiced her desire "to hurt people."

The cruel urge surfaced on May 11, 1968, when Mary and Norma Bell (no relation) were playing with a three-year-old boy on top of a Newcastle air raid shelter. The boy fell and was severely injured, but the incident was written off as accidental. On May 12, the mothers of three young girls informed police that Mary had attacked and choked their children. She was interviewed and lectured by authorities, but no juvenile charges were filed.

On May 25, two boys playing in an old, abandoned house found the corpse of four-year-old Martin Brown, lying in an upstairs room. Mary and Norma Bell had followed the boys inside, and had to be ordered out when police arrived. With no obvious cause of death, it was assumed that Martin Brown had swallowed pills from a discarded bottle, found nearby.

On May 26, Norma Bell's father caught Mary choking his 11-year-old daughter; he slapped her face and sent her home. Later that day, a local nursery school was vandalized. Police discovered notes that read "Fuch of, we murder, watch out, Fanny and Faggot," and "We did murder Martain brown, fuckof you Bastard."

Four days later, Mary Bell appeared at the Brown residence, asking to see Martin. Reminded of the tragedy, she told his grieving mother, "Oh, I know he's dead. I wanted to see him in his coffin.''

On May 31, a newly-installed burglar alarm at the vandalized nursery school brought patrolmen rushing back to the scene, where they found Mary and Norma Bell loitering beside the building. Both girls fervently denied involvement in the previous break-in, and they were released to the custody of their parents.

Two months elapsed before the disappearance of three-year-old Brian Howe, in Newcastle. An immediate search was mounted, and Mary Bell told Brian's sister that he might be playing on a heap of concrete blocks that had been dumped out on a nearby vacant lot. In fact, he was discovered there, among the tumbled slabs, but he was dead, a victim of manual strangulation, legs and stomach mutilated with a razor and a pair of scissors that police recovered at the scene. A medical examiner suggested that the killer might have been a child, since relatively little force was used.

Detectives started circulating questionnaires among the local children, asking suspects to account for their movements at the time of Brian's death. Answers from Mary and Norma Bell were inconsistent, and both girls were brought in for questioning. While Mary claimed that she had seen an older boy abusing Brian, Norma soon broke down and told of watching Mary kill the boy. At trial, in December 1968, Norma was acquitted of all charges, while Mary Bell was convicted on two counts of manslaughter.

Described by court psychiatrists as "intelligent, manipulative, and dangerous," Mary proved herself a problem inmate. In 1970, she fabricated charges of indecent assault against one of her warders, but the man was acquitted in court. In September 1977, she escaped from Moor Court open prison with another inmate, but the runaways were captured three days later. In the meantime, they had met two boys with whom they spent the night, a circumstance that placed the ego centric Mary back in tabloid headlines, offering a blow-by-blow account of how she gave up her virginity.