Typecast as a femme fatale in European arthouse and genre films, she is best regarded for her performances as Eva Kant in Mario Bava's critically re-assessed Danger: Diabolik (1968), and the dual role of Susan Dumurrier/Monica Weston in Lucio Fulci's giallo One on Top of the Other (1969).

After garnering popularity by appearing in such films as Venusberg (1963), French Dressing (1964), Masquerade (1965), Casanova 70 (1965) and Secret Agent Super Dragon (1966), Mell's attempt to launch a Broadway and Hollywood career ended with the failure of her debut musical Mata Hari.

She settled in Italy, where her high-profile love life and long association with Pier Luigi Torri, a playboy who later became one of the world's most-wanted fugitives, made her familiar to readers of tabloid press stories about the European jet set and elite Roman nightclubs.

Her other notable films during this period include Anyone Can Play (1968), Marta (1971), Ben and Charlie (1972), Seven Blood-Stained Orchids (1972), Gang War in Milan (1973), Mahogany (1975), Casanova & Co. (1977) and Mad Dog Killer (1977).

After being arrested in London over a $300 million gold mine and bank scam, and in an ironic echo of Danger: Diabolik, Torri got out of his cell, made a daring and acrobatic rooftop escape, and evaded a huge search by infuriated English police; he eventually was recaptured in America after 18 months at large.