The Sinclair Lewis Papers, which consist of manuscripts, correspondence, photographs, and personal papers, document Lewis's literary output, creative process, and the role he played in the public and intellectual life of the United States in the first half of the twentieth century. Writings in the collection feature materials for twenty-four of Lewis's novels dating from 1909 to 1951, with the bulk dating from plans for Babbit (1921) to page proofs for The Godseeker (1949). Materials include background material and notes, character sketches and plans, drafts, galley and page proofs, scripts and screenplays for theatrical and cinematic adaptations, and printed publicity and reviews. In addition to the materials for novels, there are notes, drafts, scripts, and other materials relating to seven plays and a rich variety of shorter works, including book reviews, essays, poems, short stories, and speeches.

The correspondence in the Lewis papers features single letters or small groups of letters to and from American and English writers and literary scholars and critics of the late nineteenth to mid twentieth century. Noteworthy correspondents include Sherwood Anderson, Stephen Vincent Benét, Bernard Berenson, Willa Cather, F. Scott Fitzgerald, Ernest Hemingway, William Dean Howells, Jack London, Compton Mackenzie, Thomas Mann, W. Somerset Maugham, H. L. Mencken, Upton Sinclair, Chauncey Brewster Tinker, Rebecca West, Edith Wharton, and Thornton Wilder, among others. There is also correspondence with family, friends, publishers, and organizations.

Photographs include snapshots, studio and artists' portraits, and stage scenes from plays and theatrical adaptations of novels. Photographs date from family scenes in early childhood (1894) to a portrait in Florence, Italy (1950). Studio and artists' portraits of Lewis include photographs by Robert H. Davis, Trude Fleischmann, Eric Schaal, and Carl Van Vechten. There are also photographs of Grace MacKowan Cooke, Irving Fisher, Albert Payson Terhune, Dorothy Thompson, and members of the Lewis and Kermott families.

Personal Papers feature artwork, diaries, and notes and notebooks. The artwork in the collection includes original drawings, etchings and lithographs of Lewis by Richard Hood, James Thurber, and others. The diaries, written in English and in code, date chiefly from 1900 to 1907, and include notes relating to life in high school in Sauk Centre, Minnesota and at Oberlin College and Yale University. Copies of the early diaries include transcriptions of the English-language portions of the text and translations of the coded portions of the text. The twenty-nine folders of notes and notebooks, dating from 1907 to 1950, include research for unidentified writing projects and loose manuscript material.

The Lewis papers also contain materials by and relating to Lewis from Harry E. Maule, Philip Friedman, and the Morgan Guaranty Trust Company. The Maule materials include correspondence, manuscripts, and other papers relating chiefly to the publication of Lewis's novels in the 1940s. The Friedman and Morgan Guaranty Trust Company materials consist of correspondence and financial and legal records respectively.