His next position was as writer-in-residence at Purdue University from 1965 to 1966. Soon after beginning teaching at Brown University, Cassill founded the Associated Writing Programs (now known as the Association of Writers & Writing Programs) in 1967. Cassill was appointed Associate Professor at Brown University in 1966 and then to Professor of English in 1972 where he remained until his retired from teaching as Professor emeritus in 1983. In addition to his teaching, Cassill served as U.S. Information Service lecturer in Europe from 1975 to 1976. During this time, he mounted another art exhibit in 1970. After retiring from Brown University, Cassill became the editor of "The Norton Anthology of Short Fiction", retaining this position for nearly a quarter century, until his death.