After the Eurospy film Secret Agent Super Dragon (1966), she secured the title role in the "utterly calamitous" musical Mata Hari alongside Pernell Roberts. A preview performance in Washington, D.C. became infamous for its numerous technical problems, and producer David Merrick decided to close the production. For the rest of her life, Mell had difficulty acknowledging the failure, which may have played a part in her moving to Italy where her stories of a successful Broadway run were not challenged. She said she had turned down a lucrative seven-year Hollywood contract because "the contract was a whole book. I think that even to go to the toilet I would have needed a permission." In 1968, Mell had her best-known film role as Eva Kant in Danger: Diabolik (1968), which initially was poorly received but has been championed by later film critics. At this time, Mell was appearing in what were by Italian standards major productions, which got American releases, such as Lucio Fulci's early giallo One on Top of the Other (1969). She suffered the death of a prematurely born daughter in 1969, and never had another child.