Mills was 13 when she was discovered by J. Lee Thompson, who was looking for a boy to play the lead role in Tiger Bay. She went on to take the lead in several other films, notably Whistle Down the Wind (1961).
Hayley starred in the Disney Classic, Pollyanna, a story about a young girl who moves in with her Aunt. Her Aunt and all the townspeople's lives were changed including the Pastor of the church played by Karl Malden.
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Walt Disney's wife loved her acting, and so she was contacted to participate in the Disney film The Parent Trap. She played a pair of twins who reunite their parents after a long time, and her acting earned her a special Oscar. Other Disney films in which she appeared included In Search of the Castaways, Summer Magic and .
She also acted in the film Kes, directed by Ken Loach (1969).