The Clemens-Hardaway Manuscript Collection provides very detailed genealogical and family history information on the Clemens, Mullanphy, and related families of St. Louis and elsewhere.  The earliest items in the collection, copies of original letters and legal documents made by Mary C. Clemens during her genealogical research, date to the 1770s, with the bulk of the material dating from about 1930 to 1950. Mary Clemens was engaged in extensive genealogical work as early as 1900.

Most of the collection is contained in a series entitled Genealogical Material, which includes notes, clippings, genealogical tables, historical sketches, correspondence, cemetery and church records, and copies of legal documents providing information on the Boyce, Clarke, Clemens, Mullanphy, Sherrard, and Thomas families. Another major series is that of Manuscripts, containing typescripts of Mary C. Clemens's works on the Clemens family and the descendants of John Mullanphy, which are heavily larded with annotations, notes, clippings, and correspondence. Also in this series are Raymond Martin Bell's _Ancestry of Samuel Clemens, Grandfather of Mark Twain_ and Bell's and Harriet Hardaway's _James Clemens of Washington County_. The Charts Series consists of genealogical charts for the Clemens, Mullanphy, Sherrard, and Wolfe families, along with P.O. Selby's _Mark Twain's Relatives_.

The Clippings Series is divided into three subseries covering articles on Mark Twain, the Mullanphy family, and the Clemens family. The Scrapbooks Series contains one early compilation of newspaper articles on the Clemens family and its relations dating from 1843 to 1892.

The Society of the Sacred Heart/Sisters of Charity Series contains material on the Mullanphy family's involvement with these two orders. John Mullanphy donated land to both but left the Sacred Heart gift especially hedged round with restrictions.  Included here are a legal opinion requested by Elizabeth Clemens Clarke on the identity of those Mullanphy descendants authorized to propose orphaned pupils to the Sacred Heart's Maryville Academy, and Mary C. Clemens's notes and correspondence covering discussions with the superiors of the Society concerning the education of orphans at the Academy of the Sacred Heart.

As far as possible, Mary C. Clemens's apparent original ordering of the material into categories has been maintained. This has resulted, as described above, in notes, clippings, etc. that might have become part of separate series being left within other series such as Manuscripts and the catch-all Genealogical Materials, where they were placed by Clemens.