In 1980-81 he taught as a visiting professor at the University of California, Santa Cruz. After that he began to spend an increasing amount of time in America. He was the Copland Colloquium Fellow at Amherst College in Massachusetts in 1984. He became more interested in New Music and tried to get the Hilliard Ensemble to branch out in that direction. That, and a wish to add female voices and engage in the more theatrical style of performance presentation, led to his leaving the Ensemble.
In 1990 he moved to the United States to become a professor of music at the University of California, Davis. He founded Theatre of Voices, an ensemble of male and female singers, specializing both in early and recent music, and performing in dynamic, theatrical stage presentations. Unlike the Hilliard Ensemble, it was planned to include a varying number of singers, depending on the demands of the performances. Theatre of Voices records exclusively for Harmonia Mundi USA, on which it has released a repertoire ranging from twelfth century chant to music of Arvo Pärt, including Litany of the Whale by John Cage, and the selection of early American religious music, including hymns sung in the "shaped note" tradition.
In 1996 Hillier left California to join the faculty of Indiana University, Bloomington, as director of its Early Music Institute. This is an interdisciplinary study program offering undergraduate and graduate degrees involving not just the study of ancient performance practice and musicology, but also courses in literature, fine arts, and medieval studies in other departments and schools of the university. Hillier was also conductor of Indiana University's Pro Arte Singers, its early music choir. In 2003 he became chief conductor of Ars Nova Copenhagen, and chief conductor and artistic director of the National Chamber Choir of Ireland.
Hillier has maintained his active career as a singer and conductor, appearing frequently in North America, Europe, and Japan, though he has relaxed this pace sufficiently to carry out his teaching and administrative responsibilities and also to research and write. His books include 300 Years of English Partsongs, Romantic English Partsongs, The Catch Book, and one on Estonian composer Arvo Pärt, and he has edited of a book of collected writings of composer Steve Reich.
Hillier has won many prizes for recordings. He performs on the Harmonia Mundi, ECM, EMI, Finlandia, and Hyperion labels.