In the mental health field, the creation of networks that can guarantee the smooth coordination of services and organizations across sectors is a priority in the policy agenda of several countries. In Italy, Departments of Mental Health (DMHs) have been designated responsible for the system of specialist mental health services, and also mandated as the conveners and leaders of interorganizational and cross-sectoral networks, by a system-wide reform. This study aims to understand how mental health networks have been assembled in this context and the factors and motivations that have shaped their scope. By combining an analysis of policies with a survey of DMH directors, we have determined that DMHs have preferentially formed collaborative relationships with social service providers (local governments) and the voluntary sector. In contrast, relationships with substance abuse and addiction services and primary care providers were weak and stifled by a lack of trust and by conflict about respective contributions to mental care. We explore the reasons for this selectivity in interorganizational relationships and propose that a lack of targeted incentives in policy guidelines, on the one hand, and the existence of a mandated network leadership, on the other, have led to a rather narrow range of collaborations.