Her first published book was the 1925 poetry collection "The Espalier", which was praised by A E Housman and Arthur Quiller-Couch. She was encouraged to write fiction by David Garnett. Warner's novels included "Lolly Willowes" (1926), "Mr Fortune's Maggot" (1927), "Summer Will Show" (1936), and "The Corner That Held Them" (1948). Recurring themes are evident in a number of her works. These include a rejection of Christianity (in "Mr Fortune's Maggot", and in "Lolly Willowes", where the protagonist becomes a witch); the position of women in patriarchal societies ("Lolly Willowes", "Summer Will Show", "The Corner that Held Them"); ambiguous sexuality, or bisexuality ("Lolly Willowes", "Mr Fortune's Maggot", "Summer Will Show"); and lyrical descriptions of landscape. "Mr Fortune's Maggot", about a missionary in the Pacific Islands, has been described as a "satirical, anti-imperialist novel". In "Summer Will Show", the heroine, Sophia Willoughby, travels to Paris during the 1848 Revolution and falls in love with a woman. "The Corner That Held Them" (1948) focuses on the lives of a community of nuns in a medieval convent.