Mary Stewart is the credited creator of the source material for the 2017 anime film Mary and The Witch's Flower. Born Mary Florence Elinor Rainbow on 17 September 1916 in Sunderland, England, she was a British novelist who wrote under her married name, Mary Stewart. She was a graduate of Durham University, where she earned first-class honours in English, and later worked as a lecturer in English literature at the same institution. Her professional writing career spanned from the mid-1950s to the late 1990s.

While Mary Stewart is most recognized for developing the romantic mystery genre with novels like Madam, Will You Talk? (1954) and The Moon-Spinners (1962), her connection to anime and manga comes through her work in children's literature. The 1971 children's novel The Little Broomstick serves as the direct source material for the 2017 animated film Mary and The Witch's Flower, produced by Studio Ponoc and directed by Hiromasa Yonebayashi, a former Studio Ghibli animator. This film adaptation represents her most significant credit within the Japanese animation industry.

Beyond this work, Mary Stewart was also a highly regarded author of historical fantasy, best known for her Arthurian Saga, a series of novels that began with The Crystal Cave (1970). This series retold the legend of King Arthur from the perspective of the wizard Merlin, blending historical detail with fantasy elements. Her literary career earned her numerous accolades, including the Mythopoeic Fantasy Award for The Crystal Cave and The Hollow Hills, as well as the Agatha Award for lifetime achievement in 1996.

Mary Stewart passed away on 9 May 2014. Her legacy within the context of anime is that of an original source creator, where her children's book provided the narrative foundation for a major feature film from a prominent Japanese animation studio. Her work is not that of a manga author or a direct producer of anime, but rather a British novelist whose existing literary property was adapted for the medium.