Neil Entwistle was born on September 18, 1978, near Nottingham, England. He grew up in Worksop with his parents, Clifford and Yvonne, and his younger brother, Russell. The family lived in a working-class home. Clifford worked as a coal miner, and Yvonne was a cook in a school canteen. This environment shaped Neil's early life.

Neil's education included time at the University of York, where he earned a master's degree in electronic engineering. While in university, he met Rachel Souza, an American student studying abroad. They married on August 23, 2003, in Plymouth, England. After their marriage, they moved to Worcestershire, where their daughter, Lillian, was born on April 9, 2005.

In their married life, Neil worked in computing while Rachel taught English, Drama, and Theatre Studies at a local high school. The couple eventually moved to the United States. They first stayed with Rachel's mother and stepfather in Carver, Massachusetts. Later, they found their own home in Hopkinton, which is about 26 miles from Boston.

On January 20, 2006, Neil Entwistle is believed to have killed his wife, Rachel, and their infant daughter, Lillian, in their home in Hopkinton, Massachusetts. The bodies of Rachel, 27, and Lillian, 9 months, were discovered in the master bedroom on January 22, 2006. Autopsy results showed that Rachel died from a gunshot wound to the head, while Lillian suffered a gunshot wound to the torso. The bullets involved went undetected until the autopsy revealed their presence.

After the deaths, Entwistle bought a one-way ticket to London early on January 21 and boarded a British Airways flight. His actions raised suspicion because his DNA was found on a handgun owned by his father-in-law, Joseph Matterazzo. Furthermore, DNA matching Rachel's was found on the gun's muzzle. Keys to Matterazzo's house were discovered in the car left at Logan International Airport.

Police began investigating after Rachel’s friend reported her missing. Initial searches failed to uncover the bodies, but a more thorough search revealed them the next evening. On January 23, a Massachusetts State Police trooper called Entwistle, who claimed that he left his home that morning and returned to find Rachel and Lillian shot dead. He said he covered their bodies with a blanket and did not report the incident to the authorities.

Entwistle described his extreme distress and mentioned a plan to commit suicide. Instead, he drove to his father-in-law’s house to retrieve a .22 caliber revolver but found the house locked and decided to fly back to England instead. Due to his behavior and the evidence, authorities considered him a person of interest and later issued an international arrest warrant.

On February 9, 2006, Entwistle was arrested in London after a search of his parents' home. He was extradited back to the United States on February 15, where he was charged with multiple offenses including two counts of murder. He pleaded not guilty.

In June 2008, his trial began. His defense team contested evidence collection and sought to suppress it, but the court rejected their arguments. Entwistle did not testify or call any witnesses in his defense. He was found guilty on June 25, 2008, and was sentenced to life in prison without the possibility of parole.

Following his conviction, Entwistle was transferred to different prisons for his safety due to threats against him. His conviction was automatically appealed, but the appeal was rejected in August 2012. Entwistle's parents maintain that he is innocent, asserting an alternative theory about the murders.