Cassill took a lecturing position at the University of Washington in Seattle in 1955; in 1957 he taught in New York, where he became a lecturer at both Columbia University and the New School for Social Research. Between the shift of moving from the west coast to the east coast, Cassill fell in love and married writer Karilyn Kay Adams on November 23, 1956. (An earlier marriage to artist Kathleen Rosecranz ended in divorce.) Together they had three children, Orin, Jesse, and Erica Cassill. Cassill returned to the University of Iowa in the same capacity in 1960 where he would teach for a few years at the Iowa Writer's Workshop. Among some of the students who took classes with Cassill at the Iowa Writer's Workshop during this time, and would later go on to achieve some measure of acclaim, included Clark Blaise, Raymond Carver, and Joy Williams.