Stayner was born Merced, California. His younger brother, Steven, was kidnapped by the infamous child molester, Kenneth Parnell in 1972, when Cary was 11, and held captive for more than seven years before escaping and being reunited with his family, until he was killed in a motorcycle accident while driving home from work. Cary Stayner would later say that he felt neglected while his parents grieved over Steven's death.
The following year, Stayner's uncle Jesse, was murdered. Stayner would also later claim that his uncle molested him when he was 11. In 1991, Stayner attempted to commit suicide, and was arrested in 1997 for the possession of marijuana and methamphetamine, although the charges were eventually dropped.
Stayner was hired as a handyman at the Cedar Lodge motel in El Portal, just outside the Highway 140 Arch Rock entrance to Yosemite National Park in 1997. In 1999, between February and July, he murdered two women and two teenage girls: Carole Sund; her daughter, 15-year-old Juli Sund; their travel companion, 16-year-old Argentine exchange student Silvina Pelosso; and Yosemite Institute employee Joie Ruth Armstrong, 26, a naturalist. The first two victims, Carole Sund and Silvina Pelosso, were found in the trunk of the burnt remains of Carole's Pontiac rental car. The bodies were burned beyond recognition and were identified using dental licenses. A note was found and sent to the police with a hand-drawn map indicating the location of the third victim, Juli Sund. The top of the note read, “We had fun with this one.” Investigators went to the location depicted on the map and found the remains of Juli, whose throat had been cut.
Detectives began interviewing employees of the Cedar Lodge motel where the first three victims had been staying just before their deaths. One of those employees was Cary Stayner, but he was not considered a suspect at that point because he had no criminal history and remained calm during the police interview. Upon meeting Stayner, FBI Agent, Jeff Rinek asked if Stayner had ever seen the movie Billy Jack, noting Stayner's resemblance to the film's hero. Initially, Stayner denied seeing the movie. However, 90 minutes later, after building rapport during the drive to the FBI headquarters in Sacramento from the nudist resort where he was picked up, Stayner surprised Rinek by reciting several of Billy Jack's lines from the film.
When the decapitated body of the fourth victim Joie Ruth Armstrong was found, eyewitnesses said they saw a blue 1979 International Scout parked outside the cabin where Armstrong was staying. Detectives traced this vehicle to Stayner. This caused Stayner to become the prime suspect in the case. FBI agents John Boles and Jeff Rinek found Stayner staying at Laguna del Sol nudist resort in Wilton, where he was captured and arrested. His vehicle yielded evidence linking him to Joie Armstrong. During his interrogation, Stayner confessed to the four murders as well as to sending the map for finding Juli Sund's body.
Stayner claimed after his arrest that he had fantasized about murdering women since he was seven years old, long before the abduction of his brother.
Trial and imprisonment
Stayner pleaded not guilty by reason of insanity. His lawyers claimed that Stayner had a history of sexual abuse and mental illness, manifesting itself not only in the murders, but also his OCD (obsessive-compulsive disorder). Dr. Jose Arturo Silva testified that Stayner had mild autism, obsessive-compulsive disorder, and paraphilia. In spite of this, Stayner was found sane and convicted of four counts of first degree murder.
In 2002, during the penalty phase of his trial, he was sentenced to death and thereafter entered housing in the Adjustment Center on death row at San Quentin Penitentiary in California. As of 2018, Stayner remains on death row, though San Quentin's gas chamber has been inactive since a 2006 judicial action called California executions to a halt over perceived flaws in process.