The Mason & Hanger Historical Records consist of company records that were in storage for many years and considered "historical" by the new parent company, Day & Zimmermann. As there was no order to the material, an order has been imposed that we believe is useful for access and which is essentially alphabetical by project. The projects include: Boston Traffic Tunnel, Brooklyn Battery Tunnel, Camp Zachary Taylor, Cannelton Locks and Dam, Carquinez Bridge, Fort Randall Dam, Gateway Army Ammunition Plant, Grand Coulee Dam, Harvey Canal Tunnel, Iowa Army Ammunition Plant, Kennedy Space Center, Lincoln Tunnel, Michoud Assembly Facility (for NASA), Pantex Ordnance Plant and the Van Wyck Expressway. There are also files relating to a number of other projects not listed here but which are listed in the finding aid. The company did major work for the Defense Department, the National Aeronautics & Space Administration (NASA) and the Atomic Energy Commission (AEC).

The primary grouping is the "Project Files" which have been ordered alphabetically and which sometimes contain the full range of documents relating to each project from initial bid to completion, including agreements, correspondence, photographs, drawings & blueprints, construction logs, progress reports, payroll and legal documents. The next series is "Financial & Corporate Records" that include financial reports, audits, correspondence, income-tax returns, minutes, newsletters and statements. Loose photographs, oversize material and ephemera are found at the end of the finding aid and generally relate back to "Project Files."

The collection also contains detailed records for various farm enterprises owned by Silas and H.P Mason.