Since the 1960s, documentary films have moved closer to the mainstream, thanks to the popularity of rockumentaries, association with the independent film movement, support from public and cable television, and the rise of streaming video services. Documentary films have become reliable earners at the U.S. box office and ubiquitous on streaming platforms, while historically they existed on the margins of mainstream media. How do we explain the growing commercialization of documentary films and the conditions that fueled their transformation? The growing commercialization of documentary film has not gone unnoticed, but it has not been sufficiently explained. Streaming and the growing interest in reality TV are usually offered as initial explanations whenever a documentary enters the cultural conversation or breaks a box-office record, but neither of those causes grapple with the overlapping causal mechanisms that commercialized documentary film. How Documentaries Went Mainstream provides a more comprehensive and meaningful periodization of the commercialization of documentary film. Although the commercial ascension of documentary films might seem meteoric, it is the culmination of decades-long efforts that have developed and fortified the audience for documentary features. Author Nora Stone refines rough explanations of these efforts through a robust synoptic history of the market for documentary films, using knowledge of film economics and the norms of industry discourse to tell a richer story. This periodization will allow scholars to compare the commercialization of documentary film with other genres. Drawing on archival documents, industry trade journals and popular press, and interviews with filmmakers and film distributors, Stone illuminates how documentary features have become more plentiful, popular, and profitable than ever before.