An interdisciplinary interrogation of primary evidence linking social capital and mental health sought to establish: (1) 'quality of evidence' (assessed in terms of study design, methods used to address stated questions, rigor of data analysis, and logic and clarity of interpretation of results), and (2) applicability of the evidence to public health policy and practice with respect to mental health. It is found that social capital, a complex and compound construct, can be both an asset and a liability with respect to mental health of those in receipt of and those providing services and other interventions. The most meaningful assessment of social capital or components thereof may examine individual access to rather than possession of social capital, a property of groups, and therefore an ecological variable. Theoretical advances in research on social capital serve to identify mainly two types of social capital: bonding (between individuals in a group) and bridging (between groups). Each type of social capital has cognitive and/or structural component(s) and may operate at micro and/or macro level(s). Effective mental health policy and service provision may build or strengthen bridging social capital and benefit from both bonding and bridging social capital where either or both exist. Established indicators of social capital are amenable to quantitative and qualitative assessment, preferably in tandem. However studies that employ combined research design are rare or non-existent. Interdisciplinary multi-method investigations and analyses are called for in order to unravel mechanisms whereby social capital and mental health might be meaningfully associated.