Mary Blandy was born around 1720 in Henley-on-Thames, Oxfordshire, England, as the only child of Francis Blandy, a prosperous solicitor and the town clerk of Henley. Raised in a well-to-do household, Mary received a comprehensive education, making her well-read and articulate. Her father, desiring advantageous matches for her, reportedly exaggerated her dowry to attract suitable suitors. ​

In 1746, Mary became acquainted with Captain William Henry Cranstoun, a Scottish army officer and the son of a nobleman. Despite initial approval, Francis Blandy grew suspicious upon learning that Cranstoun was already married with children in Scotland. Cranstoun insisted his marriage was invalid and sought an annulment, which he never obtained. Francis forbade the relationship, leading to tensions within the household.

Cranstoun allegedly sent Mary a powder, claiming it was a love potion to soften her father's stance. Unbeknownst to Mary, the substance was arsenic. She administered it to her father over time, leading to his deteriorating health and eventual death in August 1751. Servants, noticing her actions and the suspicious circumstances, reported her, leading to her arrest.

Mary was tried in March 1752 at the Oxford Assizes. Despite her defense that she believed the powder was harmless, the jury found her guilty of parricide. She was executed by hanging on April 6, 1752, outside Oxford Castle prison. Her case garnered significant media attention, with debates about her culpability persisting for years.