On his return to the U.S. in 1956, he joined the staff of the Freer Gallery of Art in Washington, D.C., where he served until 1965 as Curator of Chinese Art. From 1965 until his retirement in 1994 he was Professor of the History of Art at the University of California, Berkeley. The College Art Association awarded him its Lifetime Distinguished Teaching of Art History award in 1995, and at its 2004 annual meeting devoted a Distinguished Scholar session to him, with papers by colleagues and former students. In 2007 the College Art Association presented him with its Distingished Lifetime Achievement Award for Writing on Art.

His many publications include the widely-read and much-reprinted Chinese Painting (Skira, 1960) and many other books and exhibition catalogs, in addition to numerous articles on Chinese and Japanese painting that have appeared in both scholarly journals and popular publications. He was also joint author of The Freer Chinese Bronzes, vol. I, 1967. He undertook a five-volume series on later Chinese paintings, of which three volumes were published: Hills Beyond a River: Chinese Painting of the Yuan Dynasty (1976); Parting At the Shore: Chinese Painting of the Early and Middle Ming Dynasty (1978); and The Distant Mountains: Chinese Painting of the Late Ming Dynasty(1982). (The remaining two volumes will probably not be completed.) He has also published An Index of Early Chinese Painters and Paintings (1980, reprinted 2003), and, as a computer database, a similar index for Ming painting, which is unfinished. Translations of his books have been published in Chinese, Japanese, and Korean, as well as several European languages.

For the 1978-79 academic year he was Charles Eliot Norton Professor of Poetry at Harvard University, delivering a series of lectures titled The Compelling Image: Nature and Style in 17th Century Chinese Painting. These appeared in 1982 as a book, which in the following year was awarded the College Art Association’s Morey Prize for the best art history book of 1982. His Franklin D. Murphy lectures for the University of Kansas were published in 1988 as Three Alternative Histories of Chinese Painting, and the Bampton Lectures given at Columbia University appeared in 1994 as The Painter’s Practice: How Artists Lived and Worked in Traditional China. The Reischauer Lectures delivered at Harvard University in 1993 were published in 1996 as The Lyric Journey: Poetic Painting in China and Japan, and a fifth lecture series, the Getty Lectures, were presented at the University of Southern California in 1994 as The Flower and the Mirror: Representations of Women in Late Chinese Painting, They remain unpublished, but another book that grew out of them, titled Pictures for Use and Pleasure: Urban Studio Artists in High Qing China, is awaiting publication.

In 1973 he was a member of the Chinese Archaeology Delegation, the first group of art historians to visit China from the U.S., and in 1977 he returned to China as chairman of the Chinese Old Painting Delegation, which was given unprecedented access to painting collections there. Since then he has visited China frequently, lecturing at art academies and universities, organizing and participating in symposia, seeing exhibitions and collections, doing research.

He has two children by his first marriage, Nicholas, now an archaeologist and professor of ancient art at the University of Wisconsin in Madison, and Sarah, a pianist specializing in 20th century music who concertizes worldwide. Cahill and his present wife, the art historian and artist Hsingyuan Tsao, spent the 1998-99 academic year as members of the Institute for Advanced Study in Princeton. They now live in Vancouver, where she teaches at the University of British Columbia. Their twin sons Julian and Benedict were born in August, 1995. In July-August 2006, Cahill underwent a heart operation (angioplasti) and later suffered a congestive heart failure, with periods in the hospital; he is recovering from these, but is much reduced in energy and mobility.

On April 27-28. 2007 his former students organized a belated eightieth-birthday party and symposium at the Berkeley Art Museum and University of California under the title "Returning to the Shore." Orientations magazine included a tribute to him, with photos and reminiscences, in its Jan/Feb 2006 issue.