Between the 19th and the mid-20th century, the environmental movement transformed American culture, forcing a rethinking of the “Manifest Destiny” ideology that had long dominated political thinking toward an understanding of the need to protect and restore the balance between humans and nature. In 1900, there were only a few environmental movement organizations (EMOs), but by 2000, there were over 6,000 national and regional EMOs and over 20,000 local EMOs. What drove this phenomenal growth of EMOs? We examine a 100-year time series of EMO founding, showing that, in addition to the “legitimation-and-competition” effects of organizational density, EMO founding is facilitated by the discourse-creative activities of critical communities, objective threats in terms of air pollution, foundation giving, and powerful political allies in the presidency and Congress. Environmental discourses also legitimized and competed against one another, favoring “early risers” and preservationist discourse. Environmental mobilization needs to be understood in terms of the creation of new discursive frames that identify environmental problems, as well as objective environmental threats, resources, and political opportunities.