The landscape is an ideal spatial extent for managing forests because many ecological processes and disturbances occur on such scales. Moreover, landscape-level decision-making processes can improve the efficiency of forest management, as when many owners of small parcels increase the economy of scale of their operations by jointly hiring labor or selling products. Despite the potential benefits of managing at the landscape level, cooperation on management activities across property boundaries is rare among private landowners and poorly understood. We used a comparative case study approach to explain cooperative management among eight sets of individual private forest landowners in the Pacific Northwest and Upper Midwest, USA. We characterized how private forest owners cooperated on management and the outcomes they associated with cooperating, and we identified factors that influenced cooperation. We investigated whether cooperative management among private landowners may be constrained by social risks and whether formal institutions may be needed to facilitate cooperation. In the cases we investigated, owners jointly planned and implemented integrated management decisions on their collective forest properties. They perceived a number of beneficial social and ecological outcomes of cooperation. The key factors that fostered the emergence and continuity of cooperative management included shared concern, especially about risks to their properties and the health of their forests; pre-existing networks; trust; external expertise and resources; local leadership; and formal institutions. These factors are consistent with collective action and social exchange theory. Our findings shed light on social conditions that foster cooperative landscape management.