Michael Cox was born in 1956 in Monterey County, California. He was the oldest of seven children in his family. His biological father left his mother, Jean Stokes, while she was still pregnant. Afterward, Jean married a man named Forest Jayne.

Forest Jayne is believed to have abused Michael during his teenage years. In 1969, Jayne drugged and abused a boy from their neighborhood. To avoid prosecution, the family moved out of Monterey County. During a snowstorm, Jayne accidentally drove the family car off a cliff. Michael managed to rescue his mother and one of his younger brothers. While his mother was in the hospital with broken ribs, Michael took care of his siblings.

Michael went to El Dorado High School, where he worked as a student aid in the library. He also struggled with epilepsy and had an addiction to alcohol.

After finishing high school, Cox started working with a youth forestry conservation crew. During this time, he got to know Debbie and Denise Galston, both 14 years old, who lived in a foster home in Placerville. Cox had a habit of insulting Denise with hurtful words, like calling her names. On June 12, 1984, Denise was hanging out with friends at a local hangout spot called Bell Tower. Cox, driving nearby, approached 17-year-old Joanna Napoletano, who also lived in the same foster home as the Galstons. He offered to drive her to a party at a park, but once there, he made inappropriate advances towards her, causing her to run back downtown to tell Galston about the incident with Cox.

Later, as Galston was walking home, Cox approached her in his car, inviting her to another party. She agreed, and they picked up Napoletano on the way. Cox then drove them to an underpass, where he forced both girls out of the car at knifepoint. He attacked Galston, stabbing her in the throat while Napoletano managed to escape and seek help from passing cars. Despite witnessing the horrific event, Napoletano kept silent about Cox's actions. After killing Galston, Cox disposed of her body in Pollock Pines within the Eldorado National Forest.

On June 29, 1984, Cox went to a club in downtown Placerville, where he met Lynda Burrill, a 17-year-old girl. He lured her to the parking lot and then took her to a secluded area, where he brutally stabbed her to death. Then, on August 8, Cox abducted Debbie Galston, Denise's twin sister, while she was on her way to a party. Similarly, he ended her life and left her body near an oil rig.

In late July, a logging crew found part of Denise Galston's remains. Just five days later, the remains of Burrill were also discovered. This shocked the city, and investigators soon concluded that a single killer was responsible for both deaths. By early September, Debbie Galston's body had not yet been found, but some of her clothes were recovered along a highway. Eventually, her skeletal remains were found in late October and identified through dental records.

On August 12, Michael Cox was found sleeping in his car by a Placer field police officer. The officer noticed a gun sticking out from under the driver's seat, which turned out to be a fully loaded .357-caliber Smith and Wesson revolver. They also found handcuffs on the rear floorboard and a knife and scabbard in the trunk. Cox was interviewed twice about the murders but denied any involvement. However, after Joanna Napoletano provided new information, Cox was arrested on November 10.

Cox was charged with three murders on November 14, and he pleaded not guilty in January 1985. His trial took place in July, and he was found guilty. During the penalty phase, his lawyers tried to convince the jury not to give him the death sentence by highlighting his troubled childhood. Despite this, the jury sentenced him to death on August 7, and he was sent to San Quentin State Prison later that month.