Mansouri studied psychology at the University of California, Los Angeles (A.B., 1953) and was assistant professor there (1957–60). In 1957 he attended the voice program of the Music Academy of the West summer conservatory. He began his directorial career with a production of "Cosi fan tutte" at Los Angeles City College, and several musical theater productions at Marymount College. From 1960 to 1966, he worked as a resident stage director at the Zurich Opera. In his first year there, he staged new productions of "Amahl and the Night Visitors", "La traviata", "Don Pasquale", and "Samson et Dalila". From 1966 through 1976 he worked as the head stage director at the Geneva Opera. Throughout this period, he occasionally worked at numerous U.S. opera houses, including the Metropolitan Opera and many smaller companies.