Under the pseudonym Carolyn Keene, Benson wrote and published the debut Nancy Drew novel, launching one of the most enduring series in children's literature.
On April 28, 1930, the Grosset & Dunlap publishing house released The Secret of the Old Clock, the first book in what would become the phenomenally successful Nancy Drew Mystery Stories series. The book was credited to the pseudonym Carolyn Keene, a house name owned by the Stratemeyer Syndicate, but the manuscript was ghostwritten by Mildred Wirt Benson based on a plot outline provided by Edward Stratemeyer.
Benson's contribution was pivotal: she infused the character of Nancy Drew with independence, courage, and physical capability that diverged sharply from the passive, domesticated heroines typical of girls' literature at the time. Nancy Drew drove her own car, solved crimes through her own intellect, and acted without needing rescue by male characters—an unprecedented model of female agency for young readers in 1930.
Benson went on to write 23 of the first 30 Nancy Drew books, shaping the character's voice and personality more than any other author, despite not receiving public credit for decades due to the syndicate's strict pseudonym policy. The series became one of the best-selling children's book franchises in history, selling tens of millions of copies and influencing generations of readers, including future Supreme Court justices, astronauts, and other prominent women who cited Nancy Drew as a childhood inspiration.