JJoanna Dennehy was born in August 1982 in St Albans, Hertfordshire, England, and spent most of her early life in the nearby town of Harpenden. Details of her childhood suggest a troubled and unstable upbringing, marked by strained family relationships and behavioral problems that persisted into adulthood. By the time she was in her early thirties, Dennehy was the absent mother of two children, having drifted away from domestic responsibilities and into a lifestyle characterized by violence, instability, and manipulation.

Despite her role as a mother, Dennehy exhibited a deep fascination with violence and criminality. Those who later testified against her described a woman who rejected conventional norms and was drawn to chaos and destruction. Her criminal record included convictions for assault and ownership of a dangerous dog, leading to supervision by the UK Probation Service. However, oversight failures — including inexperienced probation staff — meant that the risks she posed were underestimated. At the time of her killing spree, Dennehy’s personality disorders were severe: psychiatric evaluations diagnosed her with psychopathy, antisocial personality disorder, and borderline personality disorder. These conditions, combined with her volatile lifestyle and craving for attention, laid the foundation for one of Britain’s most disturbing serial murder cases of the 21st century.

In March 2013, Dennehy embarked on a violent killing spree in Cambridgeshire, England. Her first victim was Lukasz Slaboszewski, a 31-year-old Polish national, whom she lured to her Peterborough flat between March 19 and 29, 2013, under the pretense of a sexual encounter. Upon his arrival, she stabbed him in the heart. Dennehy stored his body in a wheelie bin before disposing of it in a remote ditch. ​

On March 29, 2013, Dennehy murdered her housemate, 56-year-old John Chapman, a Falklands War veteran. She attacked him while he was sleeping, stabbing him multiple times in the neck and chest. Later that day, she killed her landlord and lover, 48-year-old Kevin Lee, stabbing him in the chest. Lee's body was discovered in a ditch near Newborough, dressed in a black sequined dress, which Dennehy had put on him to humiliate him post-mortem. ​

Following these murders, Dennehy fled to Hereford with her accomplice, Gary "Stretch" Richards. There, she randomly selected two men, John Rogers and Robin Bereza, and stabbed them in separate attacks on April 2, 2013. Both men survived despite life-threatening injuries. ​

While on remand, Dennehy plotted to escape from prison by killing or seriously injuring a prison officer, intending to use the officer's severed finger to bypass biometric security systems. This plan was discovered, and she was placed in solitary confinement from September 2013 to September 2015. Dennehy later claimed that the isolation left her "tearful and upset" and led to self-harm. However, the High Court rejected her claim that her human rights had been violated, deeming the solitary confinement necessary and proportionate given the nature of her offenses and the risk she posed. ​