The following is a list of the databases on HistoryKat that cover the United States Post Office.
- Index to Names, Aliases, and Histories of Postal Offenders: A list of the names and aliases of postal-law violators on record with the Post Office Department.
- Indexes to Rosters of Railway Postal Clerks, ca. 1883 - ca. 1902: Alphabetical lists of railway postal clerks and a list of the railway postal routes.
- Journal of Hugh Finlay: This journal was kept by the Surveyor of the Post Offices and Post Roads on the Continent of North America during his 310-mile journey from September 1773 to June 1774.
- Benjamin Franklin Ledger: This General Post Office account book was kept by Benjamin Franklin from 1770-1772 and shows some early monetary transactions of the Post Office.
- Postal entries from A Register of Officers and Agents, Civil, Military, and Naval, in the Service of the United States, 1829 and 1831: The entries are from the postal portion of the Register and include a list of postmasters as well as postal contractors.
- Record of First Returns Received from Postmasters, 1789-1832: These documents include a list of postmasters, date of appointment, and, in some cases, the rate of pay.
- Record Cards of Letter Carriers Separated from the Postal Service, 1863-1899: Images of 3x5 cards indicating the post office name, letter carrier's name, date of appointment, and date and cause of separation from employment.
- Record of Appointment of Substitute Clerks in First- and Second-Class Post Offices, 1899-1905: Registers of substitute clerks including the date of appointment.
- Record of Appointment of Substitute Mail Carriers in First- and Second-Class Post Offices, 1885-1903: Registers of mail carriers including the name, post office, and date of appointment.