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).
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 That Darn Cat.
She also acted in the film Kes, directed by Ken Loach (1969).
In 1971, she married Roy Boulting, who was many years older than her. They had a child, named Crispian Mills, and divorced in 1977. Crispian later became a singer with the band Kula Shaker and now is launching a solo career. Her second son, Jason Lawson, is the product of a 70's relationship with actor Leigh Lawson.