John Duffy was born in 1958 in the United Kingdom. He attended Haverstock School in North London, where he became close friends with David Mulcahy. Their friendship was intense, and they were often described as inseparable. Duffy later worked as a carpenter for British Rail, which provided him with knowledge of the railway system—a detail that would become significant in their crimes. He was married, but his relationship was troubled, marked by violence and control.

Between 1982 and 1986, John Duffy, often accompanied by David Mulcahy, committed a series of rapes and murders near railway stations in Southern England. Their first known murder was of 19-year-old Alison Day on December 29, 1985. She was abducted near Hackney Wick station, raped, and strangled. Her body was found weighted down in the River Lea.

On April 17, 1986, they murdered 15-year-old Maartje Tamboezer in West Horsley, Surrey. She was knocked off her bicycle, raped, and then killed. Her body was set on fire in an attempt to destroy evidence.

Their final known victim was 29-year-old Anne Lock, who was abducted on May 18, 1986, near Brookmans Park station. She was raped and murdered, and her body was discovered weeks later.

Duffy was arrested on November 7, 1986, and in February 1988, he was convicted of two murders and four rapes. He was sentenced to life imprisonment. In 1999, he confessed to additional crimes and implicated Mulcahy as his accomplice.