What was the name of Tolkien’s first completed Middle-earth story?

Tolkien’s first completed Middle-earth story was The Fall of Gondolin.

Tolkien began writing The Fall of Gondolin in 1917, while recovering from illness during the First World War. The story tells of the hidden Elvish city of Gondolin, its fall to the forces of Morgoth, and the escape of Tuor, Idril, and their son Eärendil. Tolkien regarded it as one of the foundational tales of his mythology.

The work was not published as a complete standalone book during Tolkien’s lifetime. Versions and excerpts appeared in later editorial publications, and Christopher Tolkien reconstructed and published a standalone edition in 2018. The story’s early date shows that Tolkien was developing the mythology of Middle-earth decades before The Hobbit.

The Hobbit was Tolkien’s first published Middle-earth book, but not his first completed Middle-earth story. The Children of Húrin and The Lay of Leithian belong to the same broad mythology, yet they were not the work Tolkien identified as his earliest completed tale.