The Santa Susana Mountain Park Association Collection consists of administrative records, annual reports, event calendars, environmental impact reports, feasibility studies, legal and legislative documents, maps, meeting minutes, newsletters, photographs, summaries of court proceedings and governmental hearings, and related organizational records. The collection documents SSMPA's work in lobbying to protect wilderness areas in the Simi Valley Hills, as well as the history of the stagecoach station and stagecoach trail at the foot of the Santa Susana Mountains. Copies of archaeological site reports, letters, family histories, interviews and the diary of Fabricio de la Osa (Ossa) an early resident of the San Fernando Valley are also included in the collection. The collection is divided into four major series: SSMPA Administrative Records (1968-1979), Civic, Cultural, and Social Organizations (1963-1981), Government and Public Agencies (1966-1982), and Historical Research Files (1804-1978).

Series I, SSMPA Administrative Records, consists of the administrative record of the SSMPA. The series includes agendas, correspondence, event flyers, maps, membership forms, newsletters, and organizational documents. The files are arranged alphabetically and chronologically within each subject.

Series II, Civic, Cultural, and Social Organizations, contains records relating to SSMPA's dealings with various civic, cultural, and social organizations. Documents include bulletins, correspondence, reports, membership lists, proposals and grant applications, newsletters, and project notes. The files are arranged alphabetically and chronologically within each subject.

Series III, Government and Public Agencies, documents SSMPA's work with various agencies from Los Angeles and Ventura counties, as well as several State of California departments and councils, including Parks and Recreation, the Joint Committee on Open Space Lands, and the Resources Agency. Federal agencies documented include the Department of Agriculture and the Department of the Interior. The series consists of concept reports, correspondence, court filings, environmental impact reports, legislative bills, meeting notes, notes from public hearings, petitions, and related documents. Public figures including presidents, senators, and representatives are identified within the series and generally have their own file. The files are arranged in alphabetical order under the name of branch of government and chronologically within each file.

Series IV, Historical Research Files, contains research papers and related documentation which provides evidence to establish the historical and geological significance of the Santa Susana Mountain Park Area. Accounts of early stagecoach routes and relay stations, records of early mail routes, and archaeological reports support the SSMPA's efforts to conserve this area. Documents in this series include fact sheets, landmark brochures, planning reports and proposals, and legal testimony.