Aiken was the daughter of the poet Conrad Aiken (Aiken, Conrad). While still a student, she had two poems published in The Abinger Chronicle, a prestigious little magazine, and at age 18 her first short story was published. As an adult, she wrote radio scripts and worked as a librarian for the United Nations. In 1955 she became an editor for the literary magazine Argosy and later was a copywriter for an advertising agency.

Aiken's first books, All You've Ever Wanted (1953) and More Than You Bargained For (1955), are short-story collections. The Wolves of Willoughby Chase (1962) was her first novel to combine elements of history, horror, and adventure. Set in 19th-century England, the children's book was the first in a series that included Black Hearts in Battersea (1964), The Whispering Mountain (1968), Dido and Pa (1986), and Midwinter Nightingale (2003). In 1974 Arabel's Raven was published and launched a popular series that followed the adventures of Arabel and Mortimer, her pet raven. A prolific writer, Aiken penned more than 60 short-story collections and novels for children. Her many books of adult fiction, beginning with The Silence of Herondale (1964), are categorized as terror, suspense, and mystery stories. She also wrote a number of novels based on Jane Austen (Austen, Jane)'s works, including Mansfield Revisited (1984), Jane Fairfax: Jane Austen's Emma Through Another's Eyes (1990), and Eliza's Daughter (1994). Among Aiken's nonfiction works are The Way to Write for Children (1982).