Originating from the diverse perspectives of an anthropologist, a poet, and a filmmaker, the Mass Observation project proceeded to ambitiously expand its documentation of British society, particularly during its most active period of 1937–55. The University of Sussex opened an archive of the files in 1975. Adam Matthew began microfilming portions of the archive in twelve parts in 1994.

The digital archive and its initial update includes indexed diaries, surveys, studies, full-text reports, interactive maps, and photographs from the period. More than three thousand previously unpublished pages of materials are included. The bulk of the digital archive, however, consists of finding aids to the rest of the source materials.

The collection contains two main forms of scanned documents. One is the empirical data and personal writings collected by the Mass Observation organization, the Topic Collections, and the Day Surveys. Highlights include a collection from 1939 to 1942 of approximately five hundred indexed diaries and surveys centered on a specific theme, such as dancing, shopping, and television. The second form is made up of File Reports and Publications: published and unpublished reports written on the basis of these primary sources, as well as 25 out-of-print books written by the Mass Observation organization. The digital archive also includes bibliographic summaries of data and documents from topic collections focused on dreams, radio listening, and reading habits spanning the period from 1937 to 1950. The publisher reports that the only overlap between the microfilm and the online version consists of the 6,000 pages of published documents, out of 234,000 total scanned pages of material.

Mass Observation Online is a valuable resource for cultural research in history, anthropology, sociology, literature, and gender studies, and not just specific to British society. The collection would be  especially helpful for undergraduates because it includes contextual essays describing the significance of the materials and two essays written by graduate students recounting their experiences with the archive. A CRL member noted, “Its appeal is not restricted to historians of the United Kingdom. The aim of the Mass Observation Project was to examine how modern citizens engaged in their world, the UK being merely the empirical site of the investigation.”