Hayley began her acting career unexpectedly at the age of twelve in a British crime drama called Tiger Bay (1959). It starred her father John Mills and the German film star Horst Buchholz. For her performance as ‘Gillie’ she won the Silver Bear Award at the Berlin Film Festival and a BAFTA for Best Newcomer.

A few months later Walt Disney’s wife Lillian happened to be in London and she caught a screening of Tiger Bay and thought Hayley would be perfect for the next film that Disney was planning to make called Pollyanna (1960). Walt subsequently signed Hayley to a six picture deal with the company and Mills won a ‘special’ Oscar for her performance as ‘Pollyanna’. In the film The Parent Trap (1960) she played twins Susan and Sharon. Other films included In Search of the Castaways (1962), The Moon-Spinners (1964), That Darn Cat (1965) and Summer Magic (1963).

The film Whistle Down the Wind (1961) was an adaptation of the novel written by her mother, Mary Hayley Bell. Produced by Richard Attenborough and directed by Bryan Forbes, it was his directorial debut and starred Alan Bates. The role garnered her a nomination for the BAFTA award for Best British Actress. Other films followed including: The Chalk Garden (1964) with Deborah Kerr; The Trouble with Angels (1966) with Rosalind Russell; The Family Way (1966) directed by the Boulting Brothers again playing opposite her father. She has continued to make films and loves to work in the theatre. Her television work includes The Flame Trees of Thika, Good Morning Miss Bliss and Wild at Heart.

She recently played the role of Dr. Josephine Grant in the thriller Trap written and directed by M. Night Shyamalan which premiered August 2024. She lives in London with her partner of twenty-eight years actor Firdous Bamji and close to her two sons and five grandchildren.