The series contains photographs, textual records, and original artwork spanning the period 1710-1904, predominantly 1880-1903, related to the personal lives and interests of William McLennan and his family, including interests in family and Canadian history. Additionally, a sub-series contains records related to William McLennan’s work as a writer from the 1880s-1903.

Materials include full and partial drafts of published and unpublished articles, short stories, novels, and non-fiction works; research and reference materials; incoming and outgoing correspondence; legal and official documents; collected historical materials and documents; notes; sketches and a sketchbook; journals; newspaper clippings and ephemera; photographs; and other materials. Notable correspondents include Gustavus George Stuart, Marion Paterson McLennan, William Durie McLennan, Edward William Thomson, Pemberton Paterson, John Stewart McLennan, Francis McLennan, Bartlett McLennan, Neil Stewart, Hugh McLennan, William Kingsford, and the Royal Society of Canada, among others. Legal and official documents include copies of wills, notarial exams, and a certification of graduation.

Collected historical materials include ephemeral materials such as plates of historical figures and plans of famous battle locations, as well as tracings of famous Canadian autographs, bookplates, and reproductions of historical documents. Many of the original historical documents are related to the McLennan and Stewart families, including correspondence, business and financial documents, and ephemera related to Neil Stewart (1793-1881) and legal and financial documents related to John McLennan (1789-1866). The historical documents related to John McLennan are primarily agreements for land sales in Glengarry County, Ontario, where McLennan was a conveyancer. Other historical documents do not appear to have any particular connection to the McLennan family but may have been collected out of general historical interest. These documents include correspondence, loan agreements, commissions, land grants, letters patent, and a journal / memoranda book. While many of the documents relate to individuals of limited historical importance, others are related to more notable historical subjects including François Desjordy Moreau de Cabanac, Sir John Johnson, Joseph-Maurice Lamothe, Francis Baby, Jenkins Williams, Herman Witsius Ryland, Pierre-Amable De Bonne, John Mure, William Bacheler Coltman, John Stewart, John Gray, and Thomas Dunn.