Jared Joseph Leto was born on December 26, 1971, in Bossier City, Louisiana, to Constance Leto (née 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. His father remarried, and committed suicide when Jared was eight. Leto moved frequently with his family from Louisiana to different cities around the country. "My mom's father was in the Air Force," Leto has explained, "so moving around a lot was a normal way of life." Leto has two younger half-brothers from his father's second marriage.

The same year, Leto directed the music video for "The Kill", a song by Thirty Seconds to Mars. At a press release, he stated that an "insanely obnoxious Danish albino" named Bartholomew Cubbins directed the video. He later explained his decision to direct with a pseudonym saying, "I really wanted people to be able to enjoy and experience the video without having any more preconceived notions or distractions, and just to let it be what it was. It wasn't important for me to lay claim to it in that way." Bartholomew Cubbins is a recurring character in the Dr. Seuss universe and one of Leto's favorite characters created by the Writer. "The Kill" was inspired by themes of isolation and insanity present in the Stanley Kubrick-directed psychological horror film The Shining (1980). Leto said, "The idea of isolation, identity, and self discovery were all elements present in the song. I thought this light homage was a good starting point and it soon grew to include many more elements outside of Kubrick's original piece." The short film received a largely positive response and numerous accolades, including an MTV Video Music Award. Leto next directed a short film for "From Yesterday". It was filmed in the Forbidden City and became the first ever American music video shot in the People's Republic of China in its entirety. Hundreds of costumed extras were employed for the shoot, inspired by the empires of ancient Chinese dynasties. Leto drew inspiration from Bernardo Bertolucci's historical drama The Last Emperor (1987) as well as the work of Akira Kurosawa.

In 1992, Leto moved to Los Angeles to pursue a career in directing, intending to take acting roles on the side. He found minor roles on television shows but his first break came in 1994, after he was cast opposite Claire Danes as Jordan Catalano, her love interest, in the short-lived but well-reviewed ABC teen drama My So-Called Life. The show was praised for its portrayal of adolescence and gained a strong cult following, despite being canceled after only one season. The same year, he made his television film debut starring alongside Alicia Silverstone in Cool and the Crazy, and landed his first film role in the 1995 drama How to Make an American Quilt. He later co-starred with Christina Ricci in The Last of the High Kings (1996) and got a supporting role in Switchback (1997).

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.

In 1997, Leto starred in the biopic Prefontaine in which he played the role of Olympic hopeful Steve Prefontaine. For the preparation of the role, Leto immersed himself in the runner's life, training for six weeks and meeting with members of his family and friends. He bore a striking resemblance to the real Prefontaine, also adopting the athlete's voice and upright running style. His portrayal received positive reviews from critics and is often considered his breakthrough role. Peter Stack of the San Francisco Chronicle noted how Leto played the athlete with raw vitality; "With hypnotic blue eyes and dirty blond hair, Leto captures the rock-star style Prefontaine affected, and he looks natural in fiery performances on the track, as well as off, where Pre affected a brash, confrontational style."

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 returned to acting in 2002 with the thriller Panic Room, which marked his second teaming with Director David Fincher. He played the supporting role of Junior, a burglar who terrorizes Jodie Foster. The film was well received by critics and became a financial success, grossing nearly US$200 million worldwide. In 2003, Leto and Cameron Diaz ended their four-year relationship. Leto's next film was the 2004 biographical film Alexander, directed by Oliver Stone. He portrayed Hephaestion, the closest friend of Alexander the Great. The film failed in the United States; Stone attributed its poor reception to disapproval of the depiction of Alexander's bisexuality, but it succeeded internationally, with worldwide revenue of US$167 million.

It took two years to record Thirty Seconds to Mars' second studio album A Beautiful Lie, with the band traveling to four different continents to accommodate Leto's acting career. A Beautiful Lie was released on August 30, 2005, in the United States. It has since been certified platinum by the Recording Industry Association of America (RIAA), and has reached platinum and gold status in several countries, with a sales total of over four million. The band heavily toured in support of the album and played at several major festivals, including Roskilde, Pinkpop, Rock am Ring, and Download.

Leto has been active in several charitable pursuits. He has been volunteering at the charity Art of Elysium, which helps children with serious medical conditions. He has supported the Barbara Davis Center for Childhood Diabetes, a program specializing in type 1 diabetes research and care. In 2006, he created the cover art for the album 97X Green Room: Volume 2, which proceeds from the sales benefited The Nature Conservancy. In June 2008, he joined Habitat for Humanity to work with Thirty Seconds to Mars on a home being repaired and renovated through the Greater Los Angeles Area's "A Brush With Kindness" programme. He also supported Habitat for Humanity in Sentul, Kuala Lumpur, Malaysia.

In 2007, Leto starred in the biographical film Chapter 27. He portrayed Mark David Chapman, a fanatic fan of The Beatles and the murderer of John Lennon. Leto prepared for his role by relying on interviews with Chapman and on audiotapes recorded by a librarian which the actor met during a visit to the inmate's hometown. Leto gained 67 pounds to approximate the killer's physique. The abruptness of his weight gain gave him gout. He was forced to use a wheelchair due to the stress of the sudden increase in weight put on his body. After the shooting of the film, he quickly went on a liquid diet. Chapter 27 premiered at the 2007 Sundance Film Festival. Despite divided critical opinion on the film as a whole, Leto's performance was widely praised. Owen Gleiberman of Entertainment Weekly considered his acting "a genuine transformation, as the actor submerges himself in Chapman's couch-potato flab and red-rimmed eyes. ... Leto disappears inside this angry, mouth-breathing psycho geek with a conviction that had me hanging on his every delusion."

Leto lives a vegan lifestyle and supports animal rights. In 2008, he supported the California Proposition 2 regarding treatment of farm animals. In the 2008 presidential election, Leto supported Senator Barack Obama of Illinois. In 2012, he chaired a Gen44 event, a campaign set up by Obama to energize voters under 40. Leto is a gay rights Activist. In October 2009, he raised money to the campaign against California Proposition 8, created by opponents of same-sex marriage to overturn the California Supreme Court decision that had legalized same-sex marriage. He spoke out in support of LGBT rights group Freedom Action Inclusion Rights (FAIR). In May 2012, he expressed support after hearing that Barack Obama had endorsed same-sex marriage.

In 2009, Leto recruited scores of extras and all manner of surrealist street performers for the short film of "Kings and Queens", which features a critical mass movement founded with forward-thinking and eco-conscious intentions, through Los Angeles at night. The short film premiered on November 9, 2009 at the Montalban Theater in Los Angeles and was positively received. At the 2010 MTV Video Music Awards, "Kings and Queens" received four nominations, including Video of the Year and Best Direction, and went on to win Best Rock Video. Thirty Seconds to Mars began their Into the Wild Tour in February 2010 and was among the hardest-working touring artists of the year.

After the strong relationship between Thirty Seconds to Mars and its audience, Leto launched the social media management and digital marketing company The Hive. It is based in Studio City, Los Angeles and focus on creative community building. In recent years, it has expanded to other artists such as Jessie J and Semi Precious Weapons. In 2010, Leto launched The One and Only Golden Tickets, a full Service company which operates worldwide and manages exclusive services for concerts, festivals, and events. In 2013, the company was renamed Adventures In Wonderland.

In 2011, Leto launched the online platform VyRT. Created as a live video streaming Service, it also features social networking and official merchandise. Its idea came from some frustrating experiences Leto faced with Thirty Seconds to Mars in streaming their own live events. In June 2012, VyRT was awarded Best Online Concert Experience at the O Music Awards. In 2012, Leto became an investor in Surf Air, a California-based air Service. He is also a funder for Reddit and Robinhood Markets.

After a five years hiatus from filming, Leto returned to act in the drama Dallas Buyers Club (2013), directed by Jean-Marc Vallée and co-starring Matthew McConaughey. Leto portrayed Rayon, a drug-addicted transgender woman with AIDS who befriends McConaughey's character Ron Woodroof. In order to accurately portray his role, Leto lost 30 pounds, shaved his eyebrows and waxed his entire body. He stated the portrayal was grounded in his meeting transgender people while researching the role. During filming, Leto refused to break character. Dallas Buyers Club received widespread critical acclaim and became a financial success, resulting in various accolades for Leto, who was awarded the Academy Award for Best Supporting Actor, Golden Globe Award for Best Supporting Actor – Motion Picture, Screen Actors Guild Award for Outstanding Performance by a Male Actor in a Supporting Role and a variety of film critics' circle awards for the role.

Leto has supported Aid Still Required, a non-profit corporation committed to bringing attention and humanitarian aid to areas suffering from natural disasters or human crises. After the 2010 Haiti earthquake, he released a book of photographs taken during his trip to the Caribbean country in 2011, in a bid to raise funds for the earthquake-ravaged country. He has connected with and helped various organizations since arriving there, including J/P Haitian Relief Organization. Leto spent a year in Haiti during his childhood, and returned there in January 2011 to "reconnect" with his former home following the tremor of January 2010. On September 30, 2014, Leto hosted the event Haiti: The Journey Is the Destination in New York City, benefitting those affected by the 2010 earthquake in the Caribbean country. In February 2015, Leto was named a global ambassador for World Wide Fund for Nature (WWF).

On November 3, 2015, it was announced that Thirty Seconds to Mars are working on their fifth studio album. In August 2016, the band revealed to have signed to Interscope Records and confirmed that a new album will be released in 2017. The group later unveiled that they would embark on a North American tour with Muse and PVRIS, which will take place from May to September 2017. In August 2017, "Walk on Water" was announced as the lead single from the band's fifth album. Thirty Seconds to Mars then performed the song at the 2017 MTV Video Music Awards featuring special guest Travis Scott. During the ceremony, Jared Leto received media attention for his tribute to Musicians Chester Bennington and Chris Cornell, who both died earlier that year.

In April 2016, it was announced that Leto would appear in the film The Outsider (2018), directed by Martin Zandvliet. In August 2016, Leto was cast as the villain Niander Wallace in the 2017 film Blade Runner 2049, the sequel to Blade Runner. Leto also appeared in 2036: Nexus Dawn, a promotional short film that takes place before Blade Runner 2049 directed by Luke Scott and co-starring Benedict Wong.

Following the release of Suicide Squad, Leto expressed interest in reprising his role as the Joker in Future DC Extended Universe instalments, including Ben Affleck's Batman solo film. In August 2017, it was confirmed that Leto will reprise his role in multiple films in the shared universe, including Suicide Squad 2, Gotham City Sirens and an as-of-yet untitled film centered on the Joker and Harley Quinn.