Kellar's personal papers are sub-divided into 6 sub-categories:

• 1.1 personal correspondence (1898-1962; n.d.);
• 1.2 Kellar family (1817-1961);
• 1.3 autobiographical potpourri (1905-1969);
• 1.4 financial records (1911-1952);
• 1.5 education and teaching career (1899-1916); and
• 1.6 writings and speeches (1903-1955).

Personal correspondence (1.1) is again divided. There is a general correspondence file arranged chronologically from 1898 to 1962. (There is no correspondence for the years 1899 through 1906; and there are some letters for which a date could not be ascertained.) In 1947 Kellar was awarded an LL.D. by Augustana College and Theological Seminary, Rock Island, Illinois. The correspondence (1947-1949) relating to this honor is in chronological order and filed separately in subseries 1.1.

The Kellar family file (1.2), containing the papers preserved by Kellar of his immediate family, has been arranged by family member in order of seniority, and chronologically by type of record thereunder. The first folder of the series contains whatever genealogical and family history records remain. Following this are the papers of the Reverend Mr. Isaac Kellar (1817-1843); of George Kellar, Sr., Herbert Kellar's father (1889-1893); of Mary Elizabeth Summers Kellar, H.A.K.'s mother (1876-1940); of (George) Gordon Kellar, H.A.K.'s brother (1908-1925); of Henry, June, and Steven Grimes, the family of H.A.K.'s sister June (1908-1951); of Dorothy Alderton Kellar, H.A.K.'s first wife (1906-1949); of Lucile E. O'Connor Kellar, H.A.K.'s second wife and widow (1921-1961); and of Alecea Summers Kellar Beckley, H.A.K.'s daughter and only child (1943-1951).

Isaac Kellar's papers include sermons (1818-1832); a record of baptisms and marriages in Bedford County, Pennsylvania (1820-1843); and student notebooks he kept at Princeton Seminary (1817; n.d.). The papers of George Kellar, Sr., his wife, Mary Summers Kellar, and their children, Gordon and June are not distinguishable from most family correspondence of the period. Gordon, a promising architect, was killed in a flight training accident at Kelley Field, Texas during World War I. The papers of Dorothy Alderton Kellar are among the most poignant of the collection, and are remarkable for the insight into the personality and aspirations of the young Kellar; student life at the University of Wisconsin (1911-1913); and the life of an intelligent and tragic woman reminiscent of Zelda Fitzgerald. Dorothy Kellar's life is documented from girlhood until her death from cancer in 1942. Also documented in the Kellar family file are Lucile O'Connor Kellar's years of dedication to her husband, to the McCormick Historical Association, and to her family (including a brother, Kenneth O'Connor, later (8 March 1965 - 30 June 1972) director of the Dane County Chapter of the American National Red Cross). There are a few childhood letters, cards, drawings, and records of H.A.K.'s daughter Alecea. In sum, it is a quite complete, unbroken record of a family.

The autobiographical potpourri (1.3) is a small file containing mainly, drafts of Kellar's releases to such publications as Who's Who in America; and fraternity and alumni materials (1905-1969).

Financial records (1.4) is a two-archive box file of Kellar's financial affairs. It is arranged in an alphabetical subject sequence, and chronologically thereunder from automobile expenses through bank loans, income tax, insurance, invoices and receipts, and real estate transactions.

The section containing the materials on Kellar's education and teaching career (1.5) is arranged in the order of schools attended by Kellar, or at which he taught: Sunday School at the First Presbyterian Church of Peoria, Illinois (1899-1900); Bradley Polytechnic Institute, Peoria, Illinois (1905-1906); the University of Chicago (1909); Manzanita Hall, Palo Alto, California (1909-1911); the University of Wisconsin (1911-1913); the University of Texas (1913-1914); Leland Stanford, Jr. University, and the University of Minnesota (1914-1915). Included herein are lecture notes for mediaeval history (Dana Munro and George Clark Sellery), exam questions for the Ph.D in history (1912), reading lists, syllabi, student lists and records, et cetera.

Writings and speeches (1.6) comprises 8 boxes of material including all Kellar's extant plays, articles, speeches, lectures, books, and drafts of the above. Kellar wrote prize-winning plays at the undergraduate level, lectured and presented papers at professional meetings, and had a few articles published. He was editor of the two-volume work, Solon Robinson, Pioneer and Agriculturist, contributed to the Dictionary of American Biography and several professional journals, but he did not complete either of the two books he determined early in his career to write: a history of agriculture, and “The Life and Times of Mr. Cyrus Hall McCormick.” What he did complete of the latter is filed here (box 16, folder 1; box 16, volumes 22-24).