During his years in Switzerland, Mansouri began working as guest director at various houses in Italy, including Milan's La Scala. He also worked at major opera houses in cities throughout North America, including Chicago, Houston, Philadelphia, Tulsa, San Diego, Dallas and in New York at both Metropolitan Opera and City Opera. He also directed for seven consecutive seasons at Santa Fe Opera.

In 1971, he was appointed artistic adviser to the Minister of Culture of Iran where he helped to organize Tehran Opera and staged productions through 1975. In 1976, he was named general director of the Canadian Opera Company in Toronto, an organization he brought to international recognition.