Born in Colfax, California, Riley studied at Shasta College, San Francisco State University, and the San Francisco Conservatory before earning an MA in composition at the University of California, Berkeley, studying with Seymour Shifrin. His most influential teacher, however, was the late Pandit Pran Nath, a master of Indian classical voice, who also taught La Monte Young and Marian Zazeela. Riley made numerous trips to India over the course of their association to study and to accompany him on tabla, tambura, and voice. Throughout the sixties he travelled frequently around Europe as well, taking in musical influences and supporting himself by playing in piano bars, until he joined the Mills College faculty in 1971 to teach Indian classical music.

Also during the sixties were the famous "All-Night Concerts", during which Riley performed mostly improvised music from evening until sunrise, using an old organ harmonium ("with a vacuum cleaner motor blower blowing into the ballasts") and tape-delayed saxophone. When he finally wanted a break, after hours of playing, he played back looped saxophone fragments recorded throughout the evening. For several years he continued to put on these concerts, to which people came with sleeping bags, hammocks, and their whole families.

Riley began his long-lasting association with the Kronos Quartet by meeting its founder, David Harrington, while at Mills. Over the course of his career Riley has composed 13 string quartets for it, in addition to a few other works. He wrote his first orchestral piece, Jade Palace, in 1991, and has continued to pursue that avenue, with several commissioned orchestral compositions following. Riley is also currently performing and teaching both as an Indian raga vocalist and as a solo pianist.

Musical style and techniques

While his early endeavours were influenced by Stockhausen, Riley changed direction after first encountering La Monte Young, whose Theater of Eternal Music he later performed in from 1965-66. Riley has referred to Young as "the freakiest guy I have ever met in my life," stating that it was Young's ideas that were at the heart of minimalism, though more composers have come to name Riley himself as an influence. The String Quartet (1960) was Riley's first work in this new style; it was followed shortly after by a string trio, in which he first latched on to the repetitive short phrases that he (and minimalism) are now known for.

His music is usually based on improvising through a series of modal figures of different lengths, such as in In C and the Keyboard Studies. In C (1964) is probably Riley's best-known work and one that brought the minimalistic music movement to prominence. Its first performance was given by Steve Reich, Jon Gibson, Pauline Oliveros, and Morton Subotnick, among others, and it has influenced their work and that of many others, including John Adams and Philip Glass. Its form was an innovation: the piece consists of 53 separate modules of roughly one measure apiece, each containing a different musical pattern but each, as the title implies, in C. One performer beats a steady stream of Cs on the piano to keep tempo. The others, in any number and on any instrument, perform these musical modules following a few loose guidelines, with the different musical modules interlocking in various ways as time goes on. The Keyboard Studies are similarly structured – a single-performer version of the same concept.