Joanna Dennehy was born in August 1982 in St Albans, Hertfordshire, and raised in nearby Harpenden. She grew up in a stable, middle-class family; her father, Kevin, worked as a security guard, and her mother, Kathleen, was a shop manager. Dennehy excelled academically and athletically during her school years, participating in hockey and netball teams. 

However, in her mid-teens, she began rebelling, associating with older boys, experimenting with drugs and alcohol, and eventually leaving home at 16. She entered a tumultuous relationship with John Treanor, with whom she had two children. Dennehy's behavior became increasingly erratic, involving heavy substance abuse, self-harm, and criminal activities, leading to multiple incarcerations. ​

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. ​