When he entered the Juilliard School of Music, he, like many music students, was single-mindedly concerned with the technique of his instrument, the oboe. He later urged his music students not to be so narrowly intense on a single facet of music, and joked that he flunked the mandatory piano class because he could not bring himself to practice anything but oboe. He played folk music in a jug band, but credited rock music with freeing him from the obsessive concentration on specializing on oboe. He formed The New York Rock and Roll Ensemble, a pioneer of rock/classical fusion long before there was such a name. This led him to think more about composing music, which required him to broaden the scope of his musical studies. The ensemble recorded five albums and came to the attention of Leonard Bernstein, who booked it for one of his nationally televised Young People's Concerts of the New York Philharmonic Orchestra. Bernstein, Kamen said, introduced him to scoring for symphony orchestra. Kamen was also an experienced session musician, known by several rock and popular acts by the time the Ensemble broke up in 1972. After successfully writing a ballet score, Rodin Mis en Vie, for the Harkness Ballet in 1973, Kamen went on tour with David Bowie as musical director of the rock star's Diamond Dogs tour. Kamen continued to have an interest in ballet music, composing a total of ten of them by 2000. He wrote his first film score in 1976, but during that decade primarily worked as a session musician, producer, and arranger for such groups and artists as Queen, Eurythmics, Metallica, Aerosmith, Def Leppard, Bowie, and Eric Clapton. In 1979, he collaborated with Pink Floyd to create the classic rock album The Wall.
This led to increased film work. The movie that brought him to wide attention was Terry Gilliam's wildly inventive Brazil (1983). Since that film, he went on to score over 70 more. These include all the Die Hard and Lethal Weapons movies, Highlander, The Iron Giant, What Dreams May Come, Don Juan de Marco, X-Men, 101 Dalmations, Edge of Darkness, and Mr. Holland's Opus. Along with Richard Dreyfus, who portrayed music teacher Holland in that film, Kamen set up the Mr. Holland's Opus Foundation dedicated to providing musical instruments for talented poor children. He also expanded and recorded the necessarily brief bit of music meant to portray Holland's long-delayed work into a Mr. Holland's Symphony.
His music frequently synthesized various musical genres and music from various cultures. He wrote music for the Kodo Drummers of Japan and an overture for 200 Buddhist monks, a concert (called "S & M") for symphony orchestra and the rock group Metallica (the recording of which went multi-platinum). He was nominated three times for Academy Awards and a like number of times for Golden Globes for Best Original Song in a movie.
He was chosen by the National Symphony Orchestra of Washington, D.C. to write their work in commemoration of the turn of the calendar in the form of a Millennium Symphony called The Old Moon in the New Moon's Arms.