Sixteen personal journals, seventeen letter books, two letter drafts, four additional notebooks, and several items from Mallinckrodt’s brother, nephew, and mother. Every decade from the 1860s to the 1920s is represented except for the 1890s. Materials dated before 1915 represent Mallinckrodt’s time living in St. Louis, and those after 1915 are based in Salt Lake City. Mallinckrodt was an obsessive diarist, recording his innermost thoughts and passions on even the most mundane subjects. The journals and letter books are mostly numbered by Mallinckrodt, with the numbering system starting over at least once after his move to Salt Lake City.

Personal journals from 1864-1915 provide a colorful illustration of life in postbellum St. Louis. The journals and letter books from 1915-1921 include information about life in Utah from the perspective of an outsider. He provides musings on family life, his perspective on his Salt Lake City neighbors and acquaintances, writing daily and sometimes hourly about events going on around him. Mallinckrodt occasionally lapses into German, but most text is in English. Many of the letter books are name-indexed and detail Mallinckrodt’s various business dealings. Included are letters written to Samuel Clemens, Theodore Roosevelt, and Henry Ward Beecher. Some are letterpress copy books, and others are handwritten copies or drafts of letters.

Other materials include items from Mallinckrodt’s family members such as his nephew Philip Mallinckrodt’s business diary dealing with patent law, mostly from 1931. Also of interest is a notebook of quotes, poems, and songs recorded by Mallinckrodt’s mother, Louise Mallinckrodt (1817-1893) starting in 1837. The notebook is mostly in German with some entries in English at the end.