Over the next five years, he appeared with many of the leading North American and European opera companies. In 1959, he sang the title role of "The Duke" in Donizetti's Il Duca d'Alba, a groundbreaking work produced by film legend Luchino Visconti for the Spoleta Festival. The production featured a stellar cast under the direction of Giancarlo Menotti and conductor Thomas Shippers.

In 1960, Quilico's career took another leap forward when he made his Covent Garden debut opposite soprano Joan Sutherland in Verdi's La Traviata.