In the 1960s, she would act both in films and on television, including the film, "The Rare Breed" with James Stewart and Maureen O'Hara, and on television series such as "The Man from U.N.C.L.E.", "Ben Casey" and "12 O'Clock High". The 1970s saw her working mostly in television, although she has stated that the highlight of her film career was the film "Avanti!" (1972), directed by Billy Wilder, in which she starred with Jack Lemmon, and for which she received a Golden Globe Award nomination in 1973. In 1974 Mills starred in the Italian horror film "Beyond the Door" alongside fellow English actor "Richard Johnson" playing the role of Jessica Barrett a woman who becomes demonically possessed after an unplanned pregnancy. The move was a major success making over $15 million at the box office though the producers were sued by "Warner Bros" due to similarities to "The Exorcist". Mills also appeared in a two-part 1978 episode of the TV series "The Love Boat" playing Barbara Danver, wife of Alan Danver, played by Dan Rowan, one half of the comedy duo "Rowan & Martin".