Ellroy was born in Los Angeles, California. His mother, Geneva Odelia (née Hilliker), was a nurse, and his father, Armand, was an accountant and a onetime business manager of Rita Hayworth. After his parents' divorce, Ellroy relocated to El Monte, California with his mother. When Ellroy was ten years old, his mother was murdered. The police never found the perpetrator, and the case remains unsolved. The murder, along with reading The Badge by Jack Webb (a book composed of sensational cases from the files of the Los Angeles Police Department, a birthday gift from his father), was an important event of Ellroy's youth.
Ellroy's inability to come to terms with the emotions surrounding his mother's murder led him to transfer them onto another murder victim, Elizabeth Short, the "Black Dahlia." Throughout his youth, Ellroy used Short as a surrogate for his conflicting emotions and desires. His confusion and trauma led to a period of intense clinical depression, from which he recovered only gradually.
Ellroy dropped out of school and joined the army for a short while. During his teens and twenties, he drank heavily and abused Benzedrex inhalers. He was engaged in minor crimes (especially shoplifting, house-breaking, and burglary) and was often homeless. After serving some time in jail and suffering from pneumonia, during which he developed an abscess on his lung "the size of a large man's fist," Ellroy stopped drinking and began working as a golf caddy while pursuing writing. He later said, "Caddying was good tax-free cash and allowed me to get home by 2 p.m. and write books.... I caddied right up to the sale of my fifth book."
After a second marriage in the mid-1990s to Helen Knode (author of the 2003 novel The Ticket Out), the couple moved from California to Kansas City in 1995. In 2006, after their divorce, Ellroy returned to Los Angeles. He is a self-described hermit who possesses very few technological amenities, including television, and claims never to read contemporary books by other authors, aside from Joseph Wambaugh's The Onion Field, for fear that they might influence his own. However, this does not mean that Ellroy does not read at all, as he claims in My Dark Places to have read at least two books a week growing up, eventually shoplifting more to satisfy his love of reading. He then goes on to say that he read works by Dashiell Hammett and Raymond Chandler