Riley began his long-lasting association with the Kronos Quartet when he met founder David Harringtonwhile at Mills. Over the course of his career, Riley composed 13 string quartets for the ensemble, in addition to 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. He is married to Anne Riley.
Riley's music is usually based on improvising through a series of modal figures of different lengths, such as in In C (1964) and the Keyboard Studies. The first performance of In C was given by Steve Reich, Jon Gibson,Pauline Oliveros, and Morton Subotnick. Its form was anINNOVATION: 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 the key of C. One performer beats a steady pulse 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.
In the 1950s he was already working with tape loops, a technology then in its infancy, and he has continued manipulating tapes to musical effect, both in the studio and in live performance, throughout his career. An early tape loop piece titled The Gift (1963) featured the trumpet playing of Chet Baker. Riley has composed using just intonation as well as microtones.
Riley's 1995 Lisbon Concert recording features him in a solo piano format, improvising on his own works. In the liner notes Riley cites Art Tatum, Bud Powell, and Bill Evans as his piano "heroes", illustrating the importance of jazz to his conceptions.