The son of Hollywood set designer Harry Horner, James was born August 14, 1953 in Los Angeles. When he was around ten years old his family moved to London, England, where Horner was to spend his adolesence. He studied music at the Royal College of Music, for the famous composer György Ligeti. In the 70's he and his family moved back to Los Angeles, where Horner studied at the USC, gaining a Bachelor's art degree in composition. After earning a Masters degree and a PhD in music composition and theory at UCLA he teached music theory at the same university for a couple of years.

In 1977-78 Horner composed a concert piece called Spectral Shimmers. Trying very hard getting the composition performed he finally got the Indianapolis Symphony Orchestra to perform it once, but the difficulties getting rehersal time and finding an orchestra willing to play it made him hostile to the academical world. Instead Horner started scoring student films for the American Film Institute, where he got to know Roger Corman, maker of low budget horror and sci-fi movies, and scored a couple of his movies - Battle Beyond The Stars and Humanoids From The Deep, among others.

These scores gave Horner the opportunity to score big budget studio films. First out was Star Trek II: The Wrath of Khan in 1982. The score was a hit and Horner's successful career got a splendid start. In 1983 Horner scored films such as Gorky Park, Brainstorm and the cult classic Krull.

As time went by, Horner's fame grew bigger and bigger, and he composed scores for films like Aliens (Academy Award nominated), An American Tail, Cocoon, Glory (Golden Globe nomination), Braveheart, Apollo 13 (both earning him two Academy Award nominations in 1995) and Legends of the Fall (Golden Globe nomination). But many found Horner's new scores to sound too much like his older ones. He was also accused for stealing, and borrowing, music from other composers, making Horner one of the most controversial composers in Hollywood. Horner, and his music, divided the film music world in two parts. Those who loved his music, and those who couldn't stand it.