This paper explores the effects of a group-randomized controlled trial, Community Partners in Care (CPIC), on the development of interagency networks for collaborative depression care improvement between a community engagement and planning (CEP) intervention and a resources for services (RS) intervention that provided the same content solely via technical assistance to individual programs. Both interventions consisted of a diverse set of service agencies, including health, mental health, substance abuse treatment, social services, and community-trusted organizations such as churches and parks and recreation centers. Participants in the community councils for the CEP intervention reflected a range of agency leaders, staff, and other stakeholders. Network analysis of partnerships among agencies in the CEP versus RS condition, and qualitative analysis of perspectives on interagency network changes from multiple sources, suggested that agencies in the CEP intervention exhibited greater growth in partnership capacity among themselves than did RS agencies. CEP participants also viewed the coalition development intervention both as promoting collaboration in depression services and as a meaningful community capacity building activity. These descriptive results help to identify plausible mechanisms of action for the CPIC interventions and can be used to guide development of future community engagement interventions and evaluations in under-resourced communities.