Coordinating land management across property boundaries is important in mixed-ownership forest landscapes because many forest health problems such as wildfire and invasive plants occur on scales larger than individual parcels. Despite the ecological importance of coordinated management, it is rare among private landowners. One possible explanation is that the social risks associated with coordinated management outweigh the benefits given current policies and institutions. We used a qualitative case study approach to investigate coordinated management among private landowners in the U.S. Pacific Northwest and Great Lakes regions. We characterize the social arrangements through which private forest owners pool resources and jointly plan and implement management actions, and we identify factors that contribute to the emergence and success of cooperation by private forest owners. Our findings contribute to theories of cooperation and shed light on the social conditions needed to foster cooperation by private forest owners.