Mary Ann Ansell was born in 1877 in the United Kingdom. She worked as a housemaid and was the sole domestic servant at a boarding house in Great Coram Street, Bloomsbury, London. Mary lived and slept in the kitchen of this boarding house. Her family lived in dire poverty in northeast Bloomsbury, and she had several siblings, including her sister Caroline, who was an inmate at Leavesden Asylum in Abbots Langley.​

In 1899, Mary Ann Ansell took out a life insurance policy on her sister Caroline, valued at £11.5s. Seeking to claim the insurance money, Mary baked a cake infused with phosphorus rat poison. She anonymously sent this poisoned cake to Caroline at the Leavesden Asylum. Caroline consumed the cake during tea-time and shared it with other inmates in the epilepsy ward. Tragically, Caroline died in agony, while the others who consumed the cake were fortunate to recover.

Handwriting evidence led investigators to Mary Ann Ansell. An astute Hertfordshire police detective tracked her down and arrested her at her workplace in Bloomsbury. Mary protested her innocence, stating, "I am as innocent a girl as ever was born." She was charged with murder and tried at Hertfordshire Assizes. The jury found her guilty, and she was sentenced to death by hanging. At 22 years old, Mary became the youngest woman to be hanged in the UK in the 'modern era' (after the 1868 reform act). Her execution took place at St Albans Prison on July 19, 1899.

Before her execution, there was considerable public pressure for a reprieve, citing her youth and perceived lack of mental capacity. The Metropolitan Asylums Board passed a resolution calling for clemency, and newspapers like the Daily Mail advocated for her reprieve. Over 100 Members of Parliament signed a petition to spare her life. Despite these efforts, the Home Office was unwilling to commute her sentence, viewing poisoning as a particularly heinous and premeditated crime.​