Cassini was born on January 2, 1949 to German American Harrison Müller Sr. (1929-1998) and his Italian American wife Patricia, who were professional Dancers and actors, during a tour in Woodstock, New York. She left her family at an early age and travelled abroad, working as an exotic Dancer. She had affairs with many wealthy men including Georges Simenon. She moved to Italy in 1968.

Cassini married Igor Cassini, an American Journalist of Russian and Italian nobility and brother of the Stylist Oleg Cassini in 1969. She divorced from Cassini in 1972 and moved to London with her boyfriend Greek actor Yorgo Voyagis. Their daughter Kassandra was born in 1977 and Cassini returned to cinema one year later in 1978.

Cassini obtained her first film roles by 1970, appearing in The Divorce (1970) and playing her first lead role in Piero Vivarelli's Il dio serpente the same year. She had roles in films such as Pulp (1972) and Starcrash (1978) before reaching a great success in the commedia sexy all'italiana genre; a series of films starring Lino Banfi between 1979 and 1980, namely L'insegnante balla... con tutta la classe (1979), L'infermiera nella corsia dei militari (1979), and La dottoressa ci sta col colonnello (1980).