The records of Pinkerton's National Detective Agency, the oldest company of private investigators in the United States, span the years 1853-1999, with the bulk of the material dating from 1880 to 1920. The collection documents the history of the agency founded in Chicago in the 1850s by Scottish immigrant Allan Pinkerton (1819-1884), a leading figure in crime detection, particularly for clients in business and industry. It consists of main office files from New York and Chicago and is organized into five series: Family Directors File, Administrative File, Criminal Case File, 2020 Addition, and Oversize.

The records of the Pinkerton's National Detective Agency, Inc., include the letters and papers of Allan Pinkerton, the agency's founder. These records possess a special interest to researchers as a significant portion of his papers were destroyed in the Chicago fire of 1871. While serving on the Chicago police force, Allan Pinkerton established his eponymous detective agency, which often performed work for railroad companies like the Illinois Central Railroad. In February 1861, prior to the outbreak of the Civil War, Pinkerton operatives were working on a railroad case in Baltimore and averted a suspected plot to assassinate President-elect Abraham Lincoln as he passed through the city on his way to his inauguration. Shortly afterward, General George B. McClellan asked Pinkerton (whom McClellan knew from his time as superintendent of the Illinois Central) to conduct intelligence operations for the Department of the Ohio under McClellan's command. The association continued, and, when McClellan relocated to Washington, D.C., to assume command of the Army of the Potomac, Pinkerton established a headquarters in the capital. His service on behalf of the military is represented in the form of reports from secret service agents with the Army of the Potomac, addressed to the secretary of war and the Provost Marshal Department between August 21 and November 26, 1861. (Researchers interested in Pinkerton's intelligence work for McClellan should also consult the correspondence series in the Library's George Brinton McClellan Papers for reports Pinkerton submitted to McClellan, usually signed with his pseudonym, E. J. Allen.)

After the war, Pinkerton established branches of his agency in Philadelphia and New York. George H. Bangs was in charge of the New York office for many years, and three letterbooks contain copies of correspondence written by him from February 1869 to January 1873. The nature and scope of the work done by the Pinkerton agency can be learned best perhaps from two of Allan Pinkerton's letterbooks dating from August 25, 1872, to November 12, 1883.

Following Allan Pinkerton's death in 1884, his sons William and Robert led the agency. William Pinkerton (1846-1923) headed the Chicago office, and concentrated on detective services. Robert Pinkerton (1848-1907) ran the New York office and focused on protection services, which included infiltrating labor unions and strikebreaking. After Robert's death in 1907, William steered the agency back towards detective services and investigating professional criminals, such as gamblers, jewel thieves, forgers, and bank robbers. Robert Pinkerton's son Allan Pinkerton II (1876-1930) assumed control of the New York office after his father's death, but the junior Pinkerton's death in 1930 brought his son, Robert Allan Pinkerton (1904-1967) to the helm. The final member of the Pinkerton family to lead the agency, Robert A. Pinkerton oversaw a new emphasis on providing security services. The Family Directors File includes personal and professional materials from all five of the Pinkerton men who ran the agency for over a century.

The Administrative File series selectively documents Pinkerton's business principles and practices with policy statements, procedural guidelines, and training manuals, as well as the aforementioned letterbooks. This series also contains employee records for a handful of Pinkerton employees, such as James McParland, Francis P. Dimaio, and George H. Bangs and his son George D. Bangs, who both served as general managers for the Pinkerton agency.

The Criminal Case File series comprises the largest series in the collection, but the cases included in the Pinkerton archive only represent a small fraction of the company's overall business. Many of these cases received extensive publicity years after their completion, and these files often served as a "secret archive" for professional writers to chronicle highlights of the agency's history. The resulting newspaper and magazine articles dominate the case files, although individual files may contain items created during an investigation, such as correspondence, reports from operatives, mug shots, Bertillion charts, and reward notices. Very few criminal case files contain reports compiled by operatives during investigations. As a result, researchers may need to search for operative reports in the records of the individual clients who hired the Pinkerton agency, should those personal and business archives still exist. (For example, Pinkerton surveillance reports can be found in the Benjamin F. Butler Papers in the Manuscript Division in association with an 1879 breach of promise suit brought against Senator Simon Cameron by Mary S. Oliver, an investigation which is not represented in the Pinkerton's National Detective Agency Records.)

Because the Pinkerton's National Detective Agency Records at the Library of Congress contains a limited number of records compared to the scope and duration of the agency's operations, those conducting research in the collection, especially those of a genealogical nature, are unlikely to find employee records of individuals who worked for the Pinkerton agency or were involved in a Pinkerton's investigation.

For a more detailed description of the contents of the Pinkerton's National Detective Agency Records, please consult the collection finding aid.