The correspondence files of Series I cover such subjects as ordnance contracts with Argentina, Brazil, Chile, Cuba, Italy, Russia and Turkey in the pre-World War I years (circa 1906-1913), general business correspondence and memoranda, sales letters, promotion notices, proposals and inquiries, etc. Correspondence regarding Bethlehem Steel's patent infringement suits with Midland Steel and Niles-Bement-Pond Company (1905-1909) are included. Also covered is the construction of an oil refinery in Argentina and the ore mines in Cuba, Puerto Rico, New York and New Jersey.

These topics generated large amounts of correspondence with a narrow focus. This Johnston filed by subject. In addition, Johnston corresponded with a large number of individuals and companies about a wide range of topics. These letters he filed by correspondent. Some correspondents rated their own individual file, while others, with whom Johnston corresponded infrequently, were grouped together in alphabetical folders. Series I reflects this system. Johnston filed his letters both by subject and by correspondent. Subseries A contains letters filed by correspondent, while Subseries B contains letters filed by subject.