James M. Cain met H. L. Mencken when he was a young reporter at the Baltimore Sun and Mencken was the newspaper's most famous columnist. Over the course of the next three decades, Cain would be both professionally and personally connected to the Sage of Baltimore. "Mr. M.," handwritten on seventy-seven pages of lined yellow notepaper, is Cain's longest account of his relationship with Mencken. While Cain offered Mencken biographers and researchers material both verbally and in writing, "Mr. M." appears more candid and critical. This is not surprising, for the piece as it stands would have had to be edited and typed to be accessible to a third party. If "Mr. M.," as it appears, is Cain late in life thinking to himself about the contradictions and complications of Mencken, it becomes one of the most important, and perhaps the most important, document that Cain wrote about the problem that was Mencken.