Focusing on the private governance networks of Maurice Frederick Strong and his second wife, Hanne Marstrand, this article traces the origins of corporate environmentalism and flexible interpretations of sustainable development to the 1970s, the foundational years of international environmental governance. Shifting the analytical focus from official meetings to private gatherings in the Strongs’ homes in Geneva, Nairobi, the Canadian Rockies, and Crestone, Colorado, I show that the environmental imaginaries among governance elites around Strong and Marstrand were shaped by the couple’s entanglements with industrial philanthropy, New Age spiritualism, and cybernetics. Drawing on personal archives, correspondence, and publications, I argue that the Strongs’ heterogeneous networks, which bridged counterculture environmentalism, technological optimism, and alternative approaches to international development, ultimately facilitated the growing role of corporations in environmental policymaking during the late 1980s and 1990s. This article, then, contributes to the history of global environmental governance and development by highlighting the lasting impact of private governance networks on the environmental policy landscape, exemplified by early linkages between United Nations Environment Programme (UNEP), oil industry actors, and private banking institutions.