Leto was born on December 26, 1971 in Bossier City, Louisiana. During his childhood, Leto and his family-his mother, Constance, and his older brother Shannon Leto-moved frequently, living in Louisiana, Wyoming, Colorado, Virginia, and even Haiti. After dropping out briefly in the 10th grade, Jared decided to return and focus on his education at Flint Hill School in Oakton, Virginia. He graduated from Emerson Preparatory School in Washington, DC in 1989. When he finished school, Leto enrolled at Philadelphia's University of the Arts to pursue his interests in painting. However, he then transferred to New York City's School of Visual Arts after developing an interest in acting.

In 1992, Leto moved to Los Angeles to pursue a career in music. He only took a few small part- time acting gigs to pay the bills. Two years later, he landed a role in the TV show My So-Called Life in which he played Jordan Catalano, opposite Claire Danes. After the show ended in 1995, Leto acted in several films such as How to Make an American Quilt, Girl, Interrupted, Urban Legend, The Thin Red Line, Prefontaine and Fight Club, among others. However, it was his portrayal of heroin addict Harry Goldfarb in Darren Aronofsky's Requiem for a Dream in the year 2000, that garnered him more critical praise. The same year he portrayed a misfortuned yuppie colleague of Christian Bale's Patrick Bateman, in Mary Harron's American Psycho. After that, he returned with David Fincher for Panic Room, then teamed up with Jake Gyllenhaal in Highway in 2002, then acted opposite Colin Farrell in Oliver Stone's Alexander, and later with Nicolas Cage in Lord of War. Leto is a vegetarian and especially mentions this in interviews where he talked about his increase of weight for the film Chapter 27 (alongside Lindsay Lohan) in which he plays John Lennon's killer, Mark David Chapman. He gained more than 60 pounds for this role. He starred in the film Mr Nobody playing the role of the eponymous Nemo Nobody in 2009.

Aside from his film career, Jared Leto is the lead vocalist, rhythm guitarist and sole songwriter of the band 30 Seconds to Mars along with his brother, Shannon Leto, on drums. They started the band in 1998 and have released two albums: their eponymous debut 30 Seconds to Mars and 2005's A Beautiful Lie. They began opening for other bands and started their first ever headlining tour, Forever Night Never Day, in March of 2006. In May 2007, they started touring Europe, as the opening act for Linkin Park. In August 2008, Virgin Records filed a $30 million lawsuit against Jared and 30 Seconds To Mars, claiming the band refused to deliver three albums as required by its contract. According to the lawsuit, the band "repudiated" a 1999 contract in July.

In late April 2009 the lawsuit with EMI was annulled and 30 Seconds To Mars decided to re-sign with Virgin Records. They have also promised to have their third album This Is War out sometime in 2009. Leto directed the videos for the songs "The Kill" and "From Yesterday" under the alias of Bartholomew Cubbins , and "A Beautiful Lie" under the alias of Angakok Panipaq. For most shows, Leto uses either one of two custom-made Steve McSwain guitars, a white one called "Pythagoras" or a black one called "Artemis". The guitars-one a baritone and the other with a 25 1/2" neck-have the image of a gryphon which he drew for McSwain. Occasionally he plays a white Gibson SG 61' Reissue as seen in the video for "A Beautiful Lie". Jared also play a Gibson Maestro acoustic guitar for acoustic performances. Jared uses Mesa/Boogie amps, more specifically the Triple Rectifier model and matches them with Mesa/Boogie speaker cabinets as well.

Jared and his brother Shannon Leto, who plays drums in 30 Seconds to Mars, have gotten tattoos of the first symbol of the glyphs for the band. The Arm tattoo is the Echelon "X" symbol that is part of the band iconography.