Jared Joseph Leto was born on December 26, 1971, in Bossier City, Louisiana, to Constance Leto (nee Metrejon). His mother has Cajun ancestry. "Leto" is the surname of his stepfather. His parents divorced when he was a child, and he and his older brother, Shannon Leto, lived with their mother and their maternal grandparents, Ruby (Russell) and William Lee Metrejon.

Leto formed the rock band Thirty Seconds to Mars in 1998 in Los Angeles, California with his brother Shannon. When the group first started, Jared Leto did not allow his position of Hollywood actor to be used in promotion of the band. Their debut album had been in the works for a couple of years, with Leto writing the majority of the songs. Their work led to a number of record labels being interested in signing Thirty Seconds to Mars, which eventually signed to Immortal Records.  In 1999, Leto played a gay high school teacher who attracts the attentions of Robert Downey, Jr. in Black and White, and got a supporting role in the drama Girl, Interrupted, an adaptation of the memoir of the same name by Susanna Kaysen. He then portrayed Angel Face in Fight Club (1999), a film adaptation of Chuck Palahniuk's novel of the same name, directed by David Fincher. He began dating actress Cameron Diaz in 1999 and the couple became engaged in 2000.  Leto played Paul Allen, a rival of Patrick Bateman, in the psychological thriller American Psycho (2000). Though the film polarized audiences and critics, Leto's performance was well received. The same year, he starred as heroin addict Harry Goldfarb in Requiem for a Dream, an adaptation of Hubert Selby, Jr.'s novel of the same name, directed by Darren Aronofsky and co-starring Ellen Burstyn, Jennifer Connelly, and Marlon Wayans. To prepare for his role, Leto lived on the streets of New York City and refrained from having sex for two months prior to shooting. He starved himself for months, losing 28 pounds to realistically play his heroin addict character. After the shooting of the film, Leto moved to Portugal and lived in a monastery for several months to gain weight. His performance received critical acclaim by film critics who notably praised the actor's emotional courage in portraying the character's physical and mental degradation. Peter Travers from Rolling Stone commented that Leto "excels by going beyond Harry's gaunt look to capture his grieving heart. His scenes with Ellen Burstyn as Sara, Harry's widowed mother, achieve a rare poignancy as son and mother drown in delusions."  Leto next appeared in the independent film Highway. Set in 1994, Leto is caught with the wife of his employer, a Vegas thug, and flees to Seattle with his best friend Jake Gyllenhaal in the week preceding Kurt Cobain's suicide. Filming finished in early 2000, but the film was not released until March 2002 on home video formats, although originally scheduled for a theatrical release.  During this period Leto focused increasingly on his music career, working with producers Bob Ezrin and Brian Virtue on his band's debut album 30 Seconds to Mars, which was released on August 27, 2002, in the United States through Immortal and Virgin. It reached number 107 on the US Billboard 200 and number one on the US Top Heatseekers. Upon its release, 30 Seconds to Mars was met with mostly positive reviews; music critic Megan O'Toole felt that the band managed to "carve out a unique niche for themselves in the rock realm." The album was a slow-burning success, and eventually sold two million copies worldwide.

Where was it formed at
Los Angeles,