Thor Nis Christiansen was born in Denmark. When he was five years old, his family moved to the United States. They first lived in Inglewood, California, but later moved to Solvang. In Solvang, Thor's father, Nis, opened a restaurant.

Thor was a good student when he was younger. However, things changed during his junior year of high school. He started to ignore his schoolwork and his grades dropped. Eventually, he decided to leave his parents' house and drop out of school.

After dropping out, Thor found a job working as a gas station attendant. During this period, he also gained a lot of weight. At one point, he weighed 275 pounds (125 kg).

Thor Christiansen had a gruesome method of picking up hitchhikers, shooting them in the head with a .22 caliber pistol, and then assaulting them after they were dead. His victims all looked similar, having long, straight hair, wearing the same kind of clothes, and having the same build. Because of this, the murders in Isla Vista were called the "look-alike" murders.

On April 18, 1979, Thor Christiansen picked up Lydia Preston, a 24-year-old woman, and offered her a ride. Shortly after she got into his car, he shot her in the ear. Lydia managed to escape and get to a hospital where the bullet was removed, and she survived. On July 11, 1979, Lydia saw Christiansen at a bar in Hollywood and reported him to the police. This led to his arrest.

Christiansen was taken into custody on July 27, 1979, after the police had interviewed more than 115 people over two years. He was arrested for the murders of Patricia Laney, Jacqueline Rook, and Mary Sarris and held on a $500,000 bond in a Santa Barbara County jail. The police had first noticed him in February 1977 when he was cited for being a minor in possession of liquor. At that time, they found a .22 caliber pistol in his car, but it was not until Lydia's report that they connected him to the murders.

On August 20, 1979, Christiansen was charged with the murder of Laura Benjamin, and the next day, he was charged with the attempted murder of Lydia Preston. On December 26, 1979, he pleaded "not guilty by reason of insanity," adding this to his earlier "not guilty" plea.

Christiansen's trial was scheduled to begin on February 4, 1980, in Los Angeles for the murder of Laura Benjamin and on March 17, 1980, in Santa Barbara for the Isla Vista murders. However, on March 24, 1980, he changed his plea, opting for a diminished capacity defense instead. Despite this, on April 16, 1980, he was found guilty of first-degree murder and assault after a psychiatrist testified that Christiansen had shown some degree of planning in his crimes.

During the trial, psychiatric evaluations showed that Christiansen suffered from several disorders, including "intermittent explosive disorder," a paranoid personality, chronic drug use, and necrophilia. Christiansen admitted to killing his victims, taking them to secluded places, undressing them, and sexually exploring their bodies. On May 14, 1980, he was sentenced to 25 years to life for the murder of Laura Benjamin and 9 years for the attempted murder of Lydia Preston.

Surprisingly, when the trial for the other murders began on May 28, 1980, Christiansen changed his plea to guilty for all three counts of murder. This change came just weeks after being found sane by several psychiatrists during the Los Angeles trial. On June 18, 1980, he was sentenced to life in a maximum-security prison.

On March 30, 1981, Thor Christiansen was walking in the exercise yard at Folsom State Prison when he was stabbed once in the chest. He died from his injuries, and his killer was never identified.