The collection contains both personal papers and material collected by Trewin relating to the English stage. The latter includes theatre programmes 1906-1989, theatre posters 1783-1936, photographs (original and reproduction) of productions 1905-1970, issues of theatrical periodicals 1906-1986, albums of press cuttings, and other miscellaneous material. There are also letters collected by Trewin to and from Constance and Frank Benson, and eighteen letters and postcards (three original and fifteen facsimile) from George Bernard Shaw.
The personal material includes many letters written to Trewin over the course of his career on theatrical and literary subjects. Major correspondents (with more than ten letters) include Enid Bagnold, Guy Boas, Charles Causley, Christopher Fry, Val Gielgud, Paul Scofield, Robert Speaight and Ben Travers. Other correspondence includes letters of congratulation on the presentation of Trewin's OBE in 1981, and letters of condolence written to Wendy Trewin on the death of her husband. There is also material relating to Trewin's biography of Robert Donat and a legal dispute with the playwright Veronica Haigh which arose from it. The collection contains drafts and typescripts of some of Trewin's work, over fifty notebooks, and drafts of an account by Trewin's father of his early years at sea.