John Albert Gardner III was born on April 9, 1979, in Culver City, California. When he was a child, his parents divorced. This caused him to move around a lot in Southern California. He lived in places like Lawndale, Palmdale, and Running Springs during his childhood and teenage years. Gardner later described his father as an abusive alcoholic who often beat him. His mother, Cathy Osborn, worked as a psychiatric nurse.

From a young age, Gardner faced many challenges. By the time he was six years old, he began taking psychiatric medication. At the age of ten, he spent 60 days in a psychiatric hospital. In school, he was labeled as "seriously emotionally disturbed." Gardner graduated from Rim of the World High School in Lake Arrowhead, California, in 1997. He had a grade point average of 3.2 and an IQ of 113. He also had a diagnosis of attention deficit hyperactivity disorder, or ADHD.

While he was in high school, Gardner worked various odd jobs. One of his jobs was as a lifeguard at a local resort in Lake Arrowhead. However, during his teenage years, he got into trouble with the law. He was convicted of trespassing at a high school. After finishing high school, he moved to San Diego and found a job at a Big 5 Sporting Goods store.

On February 13, 2009, 14-year-old Amber DuBois went missing in California. Her skeletal remains were discovered in March 2010, after police questioned John Albert Gardner about her disappearance. Gardner became a suspect due to his previous criminal history and behavior reported by witnesses.

Gardner's second victim was 17-year-old Chelsea King, who disappeared on February 25, 2010, while jogging near Lake Hodges. Five days later, FBI divers found her body buried in a shallow grave. Chelsea's DNA was matched to evidence found on her discarded clothing, which linked Gardner to her murder.

Gardner was arrested on February 28, 2010, during a police patrol for a man resembling him. He had a public encounter in the area where Kings' remains were later found. DNA evidence and a report of an attempted attack on another woman prior to Chelsea's murder led to his capture.

On April 16, 2010, Gardner pleaded guilty to the murders of both Amber DuBois and Chelsea King. He confessed to kidnapping, raping, and stabbing Amber, as well as to dragging Chelsea to a remote location where he raped and strangled her. Gardner also admitted to attempting to rape a third woman in December 2009.

On May 14, 2010, Gardner was sentenced to life in prison without the possibility of parole. He received two life sentences for the murders, along with additional time for prior convictions and attempted rape. Gardner is currently incarcerated at Mule Creek State Prison.