Horner was born in Los Angeles, California, in 1953 to Jewish immigrants. His father, Harry Horner, was born in Holíč, then a part of Austria-Hungary; he emigrated to the United States in 1935 and worked as a set designer and occasional art director. James’s mother, Joan Ruth (Frankel), was born into a prominent Canadian family. His brother, Christopher, is a writer and documentary filmmaker. His other brother Anthony is a doctor specializing in pediatric immunology.  James Horner started playing piano at the age of five. His early years were spent in London, where he attended the Royal College of Music. He returned to America, where he attended Verde Valley School in Sedona, Arizona, and later received his bachelor’s degree in music from the University of Southern California. After he earned a master’s degree, he started work on his doctorate at the University of California, Los Angeles (UCLA), where he studied with Paul Chihara, among others. After several scoring assignments with the American Film Institute in the 1970s, he finished teaching a course in music theory at UCLA and subsequently turned to film scoring. Horner was also an avid pilot, and owned several small airplanes.  One of Horner’s first major film scores was for the 1979 film The Lady in Red. He began his career scoring films by working for B film director and producer Roger Corman. Horner’s first composer credit was for Corman’s Battle Beyond the Stars. From there, his works gained notice in Hollywood, which enabled him to take on larger projects. Horner’s major breakthrough came in 1982, when he had the chance to score the music to Star Trek II: The Wrath of Khan; the project established him as a mainstream composer. The film’s director Nicholas Meyer quipped that Horner had been hired because the studio couldn’t afford to use the first film’s composer Jerry Goldsmith again, but by the time Meyer returned to the franchise with Star Trek VI: The Undiscovered Country, the director found that he couldn’t afford Horner either.  Horner continued composing music for high-profile releases during the 1980s, including 48 Hrs. (1982), Krull (1983), Star Trek III: The Search for Spock (1984), Commando (1985), Cocoon (1985), Aliens (1986), *batteries not included (1987), Willow (1988), Glory and Field of Dreams (both 1989).

Aliens earned Horner his first Academy Award nomination for Best Original Score in 1987, at the 59th Academy Awards. “Somewhere Out There”, which he co-composed and co-wrote with Barry Mann and Cynthia Weil for An American Tail, was also nominated that year for Best Original Song.  Throughout the 1980s, 1990s and 2000s, Horner wrote orchestral scores for children’s films (particularly those produced by Amblin Entertainment), with credits for An American Tail (1986), The Land Before Time (1988), The Rocketeer and An American Tail: Fievel Goes West (1991), Once Upon a Forest and We’re Back! A Dinosaur’s Story (all in 1993), The Pagemaster (1994), and Casper, Jumanji, and Balto (all from 1995) and Mighty Joe Young (1998) and How the Grinch stole Christmas (2000).  Horner produced no fewer than six scores during 1995, including his commercially successful and critically acclaimed works for Braveheart and Apollo 13, both of which earned him Academy Award nominations. Horner’s biggest financial and critical success would come with the score to the 1997 film Titanic. The album became the best-selling primarily orchestral soundtrack in history, selling over 27 million copies worldwide.

At the 70th Academy Awards, Horner won Oscars for Best Original Dramatic Score and Best Original Song for “My Heart Will Go On” (which he co-wrote with Will Jennings). In addition, Horner and Jennings won three Grammy Awards and two Golden Globe Awards for the soundtrack and “My Heart Will Go On”. Titanic also marked the first time in ten years that Horner worked with director James Cameron (following the highly stressful scoring sessions for Aliens, Horner declared that he would never work with Cameron again and described the experience as “a nightmare”).  Following Titanic, Horner continued to score for major productions (including The Perfect Storm, A Beautiful Mind, Enemy at the Gates, The Mask of Zorro, The Legend of Zorro, House of Sand and Fog and Bicentennial Man). Aside from scoring major productions, Horner periodically worked on smaller projects such as Iris, Radio and Bobby Jones: Stroke of Genius. He received his eighth and ninth Academy Award nominations for A Beautiful Mind (2001) and House of Sand and Fog (2003), but lost on both occasions to Howard Shore. He frequently collaborated with film director Ron Howard, a partnership that began with Cocoon in 1985.  Horner composed the 2006–2011 theme music for the CBS Evening News. The theme was introduced as part of the debut of Katie Couric as anchor on September 5, 2006.  Horner recollaborated with James Cameron on the 2009 film Avatar, which was released in December 2009 and has since become the highest-grossing film of all time, surpassing Titanic (also directed by Cameron and scored by Horner). Horner spent over two years working on the score for Avatar, and did not take on any other projects during that time. His work on Avatar earned him numerous award nominations, including his tenth Academy Award nomination as well as Golden Globe Award, British Academy Film Award, and Grammy Award nominations, all of which he lost to Michael Giacchino for Up.

Regarding the experience of scoring Avatar, Horner said, “Avatar has been the most difficult film I have worked on and the biggest job I have undertaken … I work from four in the morning to about ten at night and that’s been my way of life since March. That’s the world I’m in now and it makes you feel estranged from everything. I’ll have to recover from that and get my head out of Avatar.”  Horner also composed the score for the 2010 version of The Karate Kid, replacing Atli Örvarsson. This film—the first that Horner worked on after Avatar—was released in 2010. In 2011, Horner scored Cristiada (also known as For Greater Glory), which was released a year later, and Black Gold. In 2012 Horner scored The Amazing Spider-Man, starring Andrew Garfield. In a recent interview on his website, Horner revealed why he didn’t return to compose the second movie; that he didn’t like how the movie resulted in comparison to the first movie, and even called the movie “dreadful.” Upon his departure, he was replaced by Hans Zimmer.  At the beginning of 2015, Horner wrote the music for Jean-Jacques Annaud’s adventure film Wolf Totem, which marked his fourth collaboration with Annaud and also Horner’s first film score in nearly three years.  At the time of his death, Horner had two films he had scored but had yet to be released: Southpaw, a boxing drama film directed by Antoine Fuqua and starring Jake Gyllenhaal and Rachel McAdams, and the The 33, from director Patricia Riggen. In July 2015, a month after his death, it was discovered that Horner already had written the score for the upcoming 2016 remake of The Magnificent Seven, and had planned for it to be a surprise.  Horner’s scores have been sampled in trailers for other films. The climax of the track Bishop’s Countdown from his score for Aliens ranks fifth in the most commonly used soundtrack cues for film trailers.

On June 22, 2015, multiple international news outlets reported that Horner was presumed to have died when his S312 Tucano turboprop aircraft crashed into the Los Padres National Forest near Ventucopa, California. The following day, his representatives at the Gorfaine/Schwartz Agency posted a message on their website stating that Horner, the only person aboard the aircraft, was killed. His attorney confirmed Horner was in the airplane when it took off after fueling at Camarillo Airport. The Ventura County Medical Examiner’s Office on June 25 confirmed Horner’s death and ruled the crash an accident.  Contemporaries and collaborators around the world paid their respects to Horner, including composers Paul Williams and Alan Menken, and directors Ron Howard[46] and James Cameron. Horner was reported to be committed to the Avatar franchise; Cameron said he and Horner “were looking forward to our next gig.” Horner’s assistant, Sylvia Patrycja, wrote on her Facebook page, “We have lost an amazing person with a huge heart and unbelievable talent [who] died doing what he loved.” Many celebrities, including Russell Crowe, Diane Warren and Céline Dion, also gave their condolences. Dion, who sang “My Heart Will Go On”, one of Horner’s most popular compositions and considered Dion’s signature song, wrote on her website that she and husband René Angélil were “shaken by the tragic death” of their friend and “will always remember his kindness and great talent that changed [her] career.” Leona Lewis, who recorded Horner’s “I See You” for Avatar, said working with him “was one of the biggest moments of my life.”