The Joseph G. Dennett papers are made up materials relating to Dennett's career as a naval engineer; however, the majority of the collection consists of journals that Dennett kept while working for the United States Navy, on and off ships, and journals and record books that Dennett kept while working on Pacific Mail Steamships and at the Salem Water Works. This collection has been divided into two series.

Series I. Professional Papers consists of materials related to Dennett's career as a naval engineer for the United States Navy, Pacific Mail Steamship Company, and the Salem Water Works. The materials include an autobiography written by Dennett to his wife and daughter, detailing his career though the start of his job at the Salem Water Works and certificates and letters of recommendation. There are certificates for the degree of Master Mason from the Freemasons, one of which came in a case with a copy of the by-laws for the Golden State Chapter of San Francisco. Also included are official letters and requests, such as an order to report for duty onboard the steamer Sonoma, requests for a leave of absence, and paperwork granting Dennett his pension.

Series II. Journals and Record Books contains journals and record books that Dennett kept while working for the United States Navy, both while working in the Navy Yard and onboard ships, while working for the Pacific Mail Steamship Company, and the Salem Water Works. This series has been divided into two subseries. Subseries A. Ships' Journals and Record Books contains journals and record books in which Dennett writes about his activities on and off the ships, engineering projects he worked on, and other general information about the ship that are typically found in a ship's log. There are a number of journals for the U.S.N. steamship Kaatskill; however, there is no record of a vessel with that name. There are however records of a U.S.N. steamship Catskill that was active during the Civil War, and this is likely the vessel that Dennett was referring to; therefore, the vessel is referred to as Catskill in this finding aid. The journals for the steamships America and Great Republic (1869) had loose items between their pages that were removed from the journals, but has been placed in the same folder as the respective journals. Subseries B. Salem Water Works Journals and Record Books contains journals that Dennett kept as an engineer for the Salem Water Works. These journals include people that he met and projects that he worked on on specific days; some entries are more detailed than others. These journals are a mix of personal and business entries; in the back of some of the journals are what appear to be personal account records.