Montresor worked on the sets of nearly 30 Italian films before arriving in America in 1960. He designed operas at the Spoleto and Glyndebourne festivals. By 1963, Rudolph Bing commissioned Montresor to do the playful sets and costumes for Giancarlo Menotti's "The Last Savage" at the old Metropolitan Opera.
His 1966 white-and-gold production of "The Magic Flute" lasted in the New York City Opera repertoire for 40 years. He designed 4 Broadway shows and was nominated for a Tony Award for 3 of them.