Marshall Ashmun "Ash" Upson (1828–1894)[1] was a newspaper journalist for several years, postmaster, justice of the peace, and author.

He is notable for having ghostwritten the book The Authentic Life of Billy, the Kid, by Pat F. Garrett, 1882.

He then drifted West during the Civil War after working as a reporter for the New York Tribune.

[6] After Sheriff Pat Garrett gunned down Billy the Kid in July 1881, he asked Marshall "Ash" Upson to ghostwrite his version of the story.

For more than a century, only a few researchers questioned the wild tales contained within Upson's book.