Although McIlhenny marked only a limited amount of material as "genealogy" (or "genealogy/bio"), the topic was clearly of interest not only to him but also to his father. Letters from John D. McIlhenny's sister Isabel Nichols and cousin Selina Baillie give accounts of their Scotch and Irish ancestry, as well as a link to the Huguenot refugees who settled in Ulster in the 17th century. The most detailed accounts are the three volumes, which apparently were privately published. The McIlhenny-King and Plumer writings begin with his father's and mother's relatives who were the first to settle in the United States. The third volume chronicles the activities of his grandfather and father, and was probably intended to include later information of the McIlhenny offspring. Additional biographical information of family members is included in the clippings filed in the following "Other" subseries.