An avid reader of Baum's books and a lifelong children's writer, Thompson was born in Philadelphia, Pennsylvania and began her writing career in 1914 when she took a job with the Philadelphia Public Ledger; she wrote a weekly children's column for the newspaper. She had already published her first children's book, The Perhappsy Chaps, and her second, The Princess of Cozytown, was pending publication when William Lee, vice president of Baum's publisher Reilly & Lee, solicited Thompson to continue the Oz series. (Rumors among fans that Thompson was Baum's niece were untrue.) Between 1921 and 1939, she wrote one Oz book a year. They were all illustrated by John R. Neill, who had also illustrated Baum's Oz books, except for the first one, The Wonderful Wizard of Oz.(Thompson was the primary supporter of her widowed mother and invalid sister, so that the annual income from the Oz books was important for her financial circumstances.)

Thompson's contributions to the Oz series are lively and imaginative, featuring a wide range of colorful and unusual characters. However, one particular theme repeats over and over throughout her novels, with little variation. Typically in each of Thompson's Oz novels, a child (usually from America) and a supernatural companion (usually a talking animal), while traveling through Oz or one of the neighboring regions, find themselves in an obscure community where the inhabitants engage in a single activity. The inhabitants of this community then capture the travelers, and force them to participate in this same activity.

Another major theme has elderly characters, most controversially, the Good Witch of the North, being restored to "marriageable" age, possibly because Thompson herself never married. She had a greater tendency toward the use of romantic love stories (which Baum usually avoided in his fairy tales, with about 4 exceptions), and characters such as the athletic Peter Brown gave a "Boy's Own" quality to some of her Oz books. (While Baum's child protagonists tended to be little girls, Thompson's were boys.) She emphasized humor to a greater extent than Baum did, and always considered her work for children, whereas Baum saw no such restrictions on his intended audience.

Thompson resigned from writing Oz books in 1939 and John R. Neill took over, writing three until his death in 1942. Thompson later wrote two more Oz books, which are typically considered non-canon: Yankee in Oz (1972) and The Enchanted Island of Oz (1976). The books were primarily written in the 1950s and edited for publication. The latter was not originally written as an Oz book.

Most of these books are still under copyright and cannot legally be reproduced online.

• 1921 — The Royal Book of Oz
• 1922 — Kabumpo in Oz
• 1923 — The Cowardly Lion of Oz
• 1924 — Grampa in Oz
• 1925 — The Lost King of Oz
• 1926 — The Hungry Tiger of Oz
• 1927 — The Gnome King of Oz
• 1928 — The Giant Horse of Oz
• 1929 — Jack Pumpkinhead of Oz
• 1930 — The Yellow Knight of Oz
• 1931 — Pirates in Oz
• 1932 — The Purple Prince of Oz
• 1933 — Ojo in Oz
• 1934 — Speedy in Oz
• 1935 — The Wishing Horse of Oz
• 1936 — Captain Salt in Oz
• 1937 — Handy Mandy in Oz
• 1938 — The Silver Princess in Oz
• 1939 — Ozoplaning with the Wizard of Oz
• 1971 — Yankee in Oz
• 1976 — The Enchanted Island of Oz

A short collection of Thompson's Oz poetry, The Cheerful Citizens of Oz, was published in 1992.