The manuscripts of father William Mitchell and daughter Maria Mitchell are not easily separated, and are treated here together. The collection includes William Mitchell's autobiography, his memoir of Judge Walter Folger of Nantucket, astronomical and meteorological observations, lectures, family and other correspondence.

This is a diverse collection, including correspondence, drafts of letters, notes and notebooks (on biometric methods; tables and formulae; science, nature and method; coefficient and correlation; vitalism; Japanese language); commonplace book (1924); autobiography; and over one hundred folders of unpublished writings. The correspondence and other material covers a variety of topics, including biology, eugenics, evolution and natural selection, human heredity, paramecia, protozoa genetics, U.S. immigration policy. There is much on the Seventh International Congress of Zoology (1907); letters to his father and wives, and from students and colleagues on his seventy-fifth birthday, 1943 (1 v.); diplomas and certificates of membership; and photographs.