Marion Catherine "Kitty" Barne was born and brought up in Sussex, England. She studied at the Royal College of Music. In 1912, she married Eric Streatfeild.
She became a novelist, screenwriter and author of children's books, winning the 1940 Carnegie Medal for British children's books. She is best remembered now for her pony books Rosina Copper and its sequel Rosina and Son, about the true story of an Argentine polo pony mare. During World War II, she was a member of the Women's Voluntary Service, responsible for helping children evacuated to Sussex. She also wrote some nonfiction books, including a biography of Elizabeth Fry (1950) and a history of the Girl Guides.