The Secret of the Old Mill is a book by Leslie McFarlane, written under the pseudonym Franklin W. Dixon. It is the third book in the original Hardy Boys series, first published in 1927. This series follows two teenage brothers, Frank and Joe Hardy, who solve mysteries and engage in detective work. In The Secret of the Old Mill, the Hardy Boys investigate a counterfeiting ring operating out of an old mill. The book is written in a simple, fast-paced style typical of early 20th-century children's detective fiction and is considered a classic in young adult literature. McFarlane was one of the key ghostwriters for the Hardy Boys series, contributing significantly to its early popularity.

Charles Leslie McFarlane (October 25, 1902 - September 6, 1977) was a Canadian journalist, novelist, screenwriter, and filmmaker, who is most famous for ghostwriting many of the early books in the very successful Hardy Boys series, using the pseudonym Franklin W. Dixon.