The William Allan Neilson Papers consist of 25 linear feet of correspondence, newspaper clippings, manuscripts, press releases, lecture notes, texts of speeches, photographs, and miscellaneous papers which document Neilson's life and career. While these papers contain material dating from Neilson's years at Smith College (1917-1939), his official correspondence as president of the college is stored separately in the College Archives' holdings of records of the Office of the President.

The papers document Neilson's personal life with extensive family material. His letters to his brother and sisters are particularly valuable. Among other things these letters present Neilson's view of life at Bryn Mawr College under M. Carey Thomas, his strong opinions on world politics, his experiences tutoring Helen Keller at Radcliffe College, and his first impressions of Smith College. This series of letters is especially strong for the years between Neilson's arrival at Harvard University for graduate study in 1895 and his marriage in 1905. In addition to the Neilson family correspondence there is extensive correspondence between the Neilson and Muser families plus diaries and writings of Elisabeth Muser Neilson, her parents, and her sister. Much of the Muser family material is in German.

Other personal material such as photographs, correspondence, class notes, and student essays document Neilson's youth and education. His love of his native Scotland and his study of its language and literature are clearly in evidence in his University of Edinburgh essays.

Correspondence and miscellaneous papers, 1917-1946, document such day-to-day concerns as finances, family health, the running of the household, and travel arrangements.

Neilson's career as professor and scholar of English and Scottish literature is represented by class notes and correspondence with students and colleagues which date primarily from his years at Harvard and Radcliffe. Smith College material includes a few notes for courses Neilson taught in the English department but consists primarily of documentation of the events and celebrations of his presidency such as his appointment, inauguration, tenth and twentieth anniversaries, and retirement. Other material documents Neilson's affiliation with scholarly organizations such as the Modern Language Association, the Scottish Text Society, and the Shakespeare Association of America. There are also texts and notes for speeches on Shakespeare and Burns. Neilson's scholarly publications are represented by a number of articles, an unfinished manuscript for a book on allegory, and proofs for a 1942 edition of Shakespeare's plays and poems. Correspondence and news clippings document other writing and editorial projects in different stages of the work. Two such projects of particular interest are the 'five foot shelf' series of Harvard Classics, which Neilson edited with Charles W. Eliot, and the second edition of Webster's New International Dictionary, for which Neilson was editor-in-chief.

In addition to his work as professor, administrator, and scholar, Neilson was much in demand for committee work. As a respected educator and administrator, his advice and support was continually sought by educational institutions and organizations. Correspondence documents Neilson's work as a member of various committees for Harvard University, his suggestions on the planning of the campus and curriculum of Bennington College, and his advice and endorsements for the New School for Social Research and a variety of organizations promoting adult education such as the Affiliated Schools for Workers, the Eliot Foundation for Adult Education, and the Committee on University Extension in the Connecticut Valley.

Neilson was also well-known for his vigorous support of liberal causes of the day. His political activities are reflected in his involvement with a large number of organizations most of which worked for civil liberties, peace, aid to refugees, or improved foreign relations. This work is documented by correspondence with such organizations as the American Civil Liberties Union, the Foreign Policy Association, the American Society for Cultural Relations with Russia, the National Refugee Service, various League of Nations organizations, and the Commission to Study the Organization of Peace.

All of Neilson's interests and concerns are in evidence in his speeches and writings. Texts contained in the papers are a rich source for the study of life at Smith College, and trends in education and politics during Neilson's term as president of Smith (1917-1939). News clippings document the quite considerable attention paid to his views.

Neilson was a renowned speaker who seldom prepared a text, preferring to talk from cursory notes made shortly before the speech was to be delivered. Because of this process the texts contained in the papers are almost exclusively transcriptions. Some of them were edited for publication, others remain more or less as they were delivered. The papers also contain some of Neilson's handwritten notes for speeches.

Correspondence, citations, and news clippings document Neilson's many awards and honors and a group of biographical articles gives an overview of Neilson's life and work. The papers also contain notes, typescripts, and correspondence for Margaret Farrand Thorp's 1956 biography Nelson of Smith.