The bulk of the collection is comprised of scrapbooks dating from 1904-09, the years of McClellan's tenure as New York City mayor. Most of these scrapbooks hold only newspaper clippings and are in their original arrangement, which is broadly topical. These topics are "political," (22 volumes), "editorial: political & department," (9 volumes), "department" (28 volumes), "legislature," (5 volumes), and "political cartoons" (7 volumes). Political clippings are generally those that concern elections, party politics, candidates, and other partisan political matters. Department clippings relate to city agencies, bureaus, and other administrative and policy matters. Legislature clippings relate to New York state legislative concerns. The title of some albums refer to editorials, but these can be found in the other albums as well. The albums titled as cartoons hold those, but general clippings also. Each album is indexed and their clippings are captioned.

In addition to the clippings scrapbooks, there are 6 volumes of the "Mayor's Flimsey Books," so called likely because most of the documents in these albums are on fragile, tissue-like paper. These documents include copies of typed speeches, outgoing letters, incoming reports, mayoral veto statements, budgets, appointments, and other important documents tracing McClellan's mayoralty and political positions. Rapid transit, city governance, and street gas lighting are among the many topics reflected in the flimsies. The flimsey albums are indexed and the documents captioned.

Also related to McClellan's mayoralty is a bound volume of the published Supplement to the City Record of July 1909, with a comprehensive list of city officials and employees, and a scrapbook of lists and other arrangements for the ceremonies of the opening of the Willamsburg Bridge in 1903, which included mayor-elect McClellan.

The collection also includes 6 volumes of clippings documenting McClellan's career from 1890-1903, that is, before he became mayor, and one volume of reviews of his 1904 book "The Oligarchy of Venice."

Finally, the collection includes 3 volumes of clippings concerning McClellan's father, the Civil War general of the same name. Some of these clippings date from the 1860s, but most seem to date from around the elder McClellan's death in 1885 and later.