The first Nancy Drew book, "The Secret of the Old Clock," was published in 1930 and was written by Mildred Wirt Benson.
After Benson's initial success, the series continued under the pseudonym, with many different authors contributing to the series over the years.
The decision to use a pseudonym was likely due to the common practice at the time of using pen names for series books, especially those targeting young readers. This allowed for a consistent brand and prevented the public from focusing on the individual authors.