Following her departure from Disney, Mills continued her film career with a series of diverse roles. For Universal Pictures, she starred alongside her father, John Mills, in
The Truth About Spring (1965), which also featured Disney regular James MacArthur. While the film achieved modest success, her next role was in
The Trouble with Angels (1966), which proved to be a major box office hit. In the film, directed by
Ida Lupino, Mills portrayed a mischievous student at a Catholic boarding school opposite
Rosalind Russell. That same year, she provided the voice of the Little Mermaid in the Rankin/Bass animated feature
The Daydreamer (1966). Mills subsequently starred in
The Family Way (1966), directed by
Roy Boulting. The film, which explored the challenges of a newlywed couple, was noted for its mature themes and featured a musical score by
Paul McCartney with arrangements by
Beatles producer
George Martin. During the production, Mills began a romantic relationship with Boulting; the two married in 1971. In 1967, Mills appeared in
Pretty Polly, a romantic drama set in
Singapore, opposite Indian film star
Shashi Kapoor. She next starred in the psychological thriller
Twisted Nerve (1968), reuniting with
The Family Way co-star
Hywel Bennett. The controversial film, directed by Boulting, attracted attention for its unsettling themes. She followed this with the romantic comedy
Take a Girl Like You (1970), co-starring
Oliver Reed, and made her West End stage debut in a 1970 production of
The Wild Duck. Mills continued her collaboration with Boulting in
Mr. Forbush and the Penguins (1971), stepping into the lead female role after the original actress was replaced during production. In 1972, she again starred opposite Bennett in
Endless Night, a mystery-thriller based on the novel by
Agatha Christie, featuring
Britt Ekland,
George Sanders, and
Per Oscarsson. Throughout the mid-1970s, Mills took on roles in several British and international productions, including
What Changed Charley Farthing? (1974),
Deadly Strangers, and
The Kingfisher Caper (both 1975), the latter co-written by Boulting. Following
The Kingfisher Caper, Mills significantly reduced her feature film work but continued to act in occasional roles over the ensuing decades. She appeared as Miss Quinton in the television adaptation
Appointment with Death (1988), and as Sally Ryan in the horror anthology film
After Midnight (1990). In 1994, she contributed her voice to the animated feature
A Troll in Central Park, portraying the character Hillary. In the 2000s, Mills took part in independent productions, including
2BPerfectlyHonest (2004), in which she played Terri, and the short film
Stricken (2005), where she portrayed Hildy. She later appeared in the family adventure
Mandie and the Cherokee Treasure (2010) as Mary Elizabeth Taft, adapted from the
Mandie book series, and in the drama
Foster (also known as
Angel in the House, 2011) as Mrs. Lange. Mills continued to appear sporadically in film thereafter. In 2021, she played Celia Towers in the time-travel drama
Last Train to Christmas, and in 2023, she portrayed Karen Walters in
Arthur’s Whisky. In 2024, Mills appeared in a major American theatrical release in a supporting role as FBI profiler Dr. Josephine Grant in
M. Night Shyamalan's psychological thriller
Trap. Her casting in the film was highlighted by Shyamalan and in media coverage as a notable and unexpected return for the actress.
Trap marked Mills's first major role in a mainstream American film in nearly 60 years, following her performance in
The Trouble with Angels (1966). Mills's post-Disney career was marked by a conscious shift toward more adult and diverse roles. As one critic observed, "She was a movie star for about a decade... a genuine, old-school, above-the-title movie star: listed in box-office polls, the focus of a carefully-protected public image, signatory to a long-term contract with a studio who would try to craft vehicles for her. In fact, you could make an argument that Hayley Mills was one of the last stars for whom that last factor applied, at least in English-speaking cinema". ==Television resurgence and reception==