John Thomas Straffen was born on February 27, 1930, at Bordon Camp in Hampshire, England. His father was a soldier in the British Army. John was the third child in his family. He had an older sister who was thought to have significant mental challenges and passed away in 1952. When John was two years old, his father was posted to British India, and the family lived there for six years. In March 1938, they returned to the United Kingdom when his father left the Army and they moved to Bath, Somerset.

In October 1938, John started having trouble. He was sent to a child guidance clinic because of stealing and skipping school. In June 1939, he went to juvenile court for stealing a girl's purse and received two years of probation. His probation officer noticed that John did not understand right from wrong. During this time, John's family lived in cramped conditions, and his mother couldn't give him much attention. This led the probation officer to take John to see a psychiatrist. He was later declared mentally defective under the Mental Deficiency Act 1927. A report from 1940 assessed John's IQ as 58, meaning he had a mental age of around six years old.

In June 1940, John was sent to a residential school for children with mental disabilities. This school was called St Joseph's School, located in Sambourne, Warwickshire. Two years later, he moved to a senior school called Besford Court. There, he was seen as a lonely boy who did not react well to discipline. At the age of 14, he was suspected of strangling two geese. When he turned 16, school authorities reviewed his case and found that his IQ had increased to 64, with a mental age of around nine and a half years. They recommended that he be discharged from the school.

After leaving school, John returned to Bath in March 1946. He received another examination from a Medical Officer of Health, who confirmed he was still considered mentally defective. John tried various short jobs and eventually found work as a machinist in a clothing factory. However, he began stealing small items from unoccupied homes and hiding them. He had no friends and acted on his own. John had several incidents that became known to the police, but his life before he became known for more serious crimes was marked by these early challenges and incidents that highlighted his troubled mental state.

On July 15, 1951, John Straffen killed a five-year-old girl named Brenda Goddard. He encountered Brenda while she was picking flowers near her home in Bath. Straffen offered to lead her to a better spot. After lifting her over a fence, he attacked her. He strangled her and left her body in a nearby copse before going to the cinema to watch a film.

Police began to suspect Straffen after the murder and interviewed him on August 3. He became aware of this and expressed a desire to annoy the police. On August 8, Straffen murdered another girl, nine-year-old Cicely Batstone. He took Cicely to a meadow after first watching a movie. There, he strangled her as well. Several people witnessed him with Cicely that day, leading to his swift identification as a suspect.

Straffen was arrested on the morning of August 9. He confessed to killing both girls during police questioning. He was charged with their murders and taken into custody. During his trial on October 17, 1951, a jury found him unfit to plead due to his mental state and he was sent to Broadmoor Hospital.

On April 29, 1952, Straffen escaped from Broadmoor. Later that day, he murdered another young girl, five-year-old Linda Bowyer, as she rode her bicycle in Farley Hill. He was captured shortly after. Police questioned him about Linda's murder. Straffen denied the crime but showed knowledge of the event without being prompted.

He was subsequently charged with Linda's murder and tried again on July 21, 1952. The trial included evidence of his earlier murders. The jury found him guilty and declared him sane, sentencing him to death. His death sentence was overturned when Home Secretary David Maxwell Fyfe recommended a reprieve, leading to a life sentence instead.

After his reprieve, Straffen was transferred to several prisons over the years. He remained in custody until his death on November 19, 2007, at the age of 77. He had been in prison for over 55 years, the longest-serving prisoner in British history.