Herbert William Mullin was born on April 18, 1947, in Salinas, California. His father was known to be stern but not abusive. Shortly before Mullin's fifth birthday, the family relocated to San Francisco.

In school, Mullin had many friends and was even voted "Most Likely to Succeed" by his classmates at San Lorenzo Valley High School when he was 16. Despite this, he began experiencing difficulties due to paranoid schizophrenia. After graduating from high school in 1965, Mullin was devastated by the death of his friend Dean Richardson in a car accident. This loss led him to build "shrines" to Richardson in his room and become obsessed with the idea of reincarnation.

In 1969, Mullin was admitted to Mendocino State Hospital. Over the following years, he was admitted to several mental hospitals but was discharged each time, as he was deemed no threat to himself or others. Altogether, he was committed to five mental hospitals. By his mid-twenties, Mullin had a diagnosis of paranoid schizophrenia, which was exacerbated by his use of LSD and cannabis.

By 1972, Herbert Mullin, aged 25, had moved back in with his parents in Felton, California, located in the Santa Cruz Mountains. His birthday, April 18, coincided with the anniversary of the 1906 San Francisco earthquake, a date he found highly significant.

Mullin believed that the Vietnam War had caused enough American deaths to prevent earthquakes as a blood sacrifice to nature. However, with the war winding down by late 1972, he felt he needed to start killing people to avert a catastrophic earthquake. He claimed that his father, through telepathy, had instructed him to take lives.

On October 13, 1972, Mullin killed 55-year-old Lawrence "Whitey" White by beating him with a baseball bat. White had approached Mullin's car, believing it was in need of repair, and offered assistance in exchange for a ride. Mullin dragged White's body into the woods, where it was discovered the next day. Mullin later said White resembled Jonah from the Bible and sent telepathic messages urging him to be sacrificed.

Mullin committed his second murder on October 24, 1972. He offered a ride to Mary Margaret Guilfoyle, a Cabrillo College student, and stabbed her in the chest while driving. He later disemboweled her body to examine her organs for pollution evidence. Her remains were found months later in February 1973.

Doubts about his hallucinations led Mullin to seek confession at St. Mary's Catholic Church in Los Gatos on November 2, 1972. While in the confessional, he claimed Father Henri Tomei volunteered to be his next sacrifice. Mullin killed Tomei on the spot before fleeing.

In January 1973, Mullin attempted to join the United States Marine Corps to legally carry out his mission but was rejected due to his refusal to sign his criminal record.

By 1973, Mullin had ceased drug use entirely and blamed his past troubles on substance abuse. Seeking retribution, he decided to confront Jim Gianera, a former friend who had introduced him to cannabis. On January 25, 1973, Mullin went to Gianera's home, shot him and his wife, and then returned to the home of their mutual friend, Kathy Francis. There, he killed Francis and her two children, believing them to be connected to his downfall.

On February 10, 1973, Mullin encountered four teenage boys camping illegally in Santa Cruz. He posed as a park ranger and told them to leave, but they ignored him. The next day, Mullin returned and shot all four in the head. He took their rifle and $20 before leaving.

Mullin's final murder occurred shortly thereafter. He shot 72-year-old Fred Abbie Perez, a retired prizefighter, while Perez was working in his garden. This killing was witnessed by a neighbor, leading to Mullin's arrest.

Despite the seemingly random nature of the murders, police did not initially connect them due to differences in weapons, methods, and victim profiles. Mullin's actions overlapped with those of another serial killer, Edmund Kemper, further complicating the investigation.

Mullin's trial began on July 30, 1973. He admitted to the crimes, and the focus was on his sanity. Prosecutors argued he understood his actions, while the defense claimed his delusions drove him to kill. Mullin was found guilty of ten murders, with some classified as first-degree and others as second-degree.

In December 1973, Mullin pled guilty to the murder of Father Tomei and was sentenced to life imprisonment. He was diagnosed with paranoid schizophrenia and remained incarcerated at Mule Creek State Prison. Mullin died on August 18, 2022, at the age of 75 due to natural causes.