The Dayton Papers consist, for the most part, of correspondence and documents illustrative of the role of Dayton as U.S. minister to France (1861-1864) during most of the Civil War. Included are diplomatic dispatches, legation accounts, personal correspondence, documents pertaining to household affairs in Paris (insurance policies, leases, etc.), and material relating to his sudden death. In addition, there is a box of others' papers, most of which contains correspondence written to and by his son, William Lewis Dayton Jr., Class of 1858, who served as second secretary in the Legation of Paris during his father's lifetime and who was later himself minister to the Hague 1882-1885, together with a few papers concerning other members of the family.

An item of historical interest is William L. Dayton, Jr.'s eye-witness account (in a 30-page letter) of the naval battle (June 19, 1864) between the Union steamer Kearsarge and the Confederate steamer Alabama off the coast of Cherbourg, France, which he observed from shore after delivering a dispatch to the Kearsarge's Captain Winslow. There are also catalogs of Dayton's law and general libraries in Trenton, New Jersey.