Family relationships account for much of the support available to individuals with mental illness. Although some studies have acknowledged the importance of family support, and while others have underlined the harmful effects of negative relationships, research has seldom empirically considered the complex web of positive and negative relationships in family networks. This research hypothesised that social capital has distinct consequences for psychological health depending on the presence or absence of negative family relationships. Through a five-wave follow-up of 60 individuals undergoing psychotherapy in a private practice, the study explored the structural features of positive and negative relationships, considered jointly, in the family networks of adults with mental illness. Four patterns of relationships were found: bonding social capital, bridging social capital, overload and ego-centred conflict. Compared to individuals within a bonding or bridging social capital pattern, those experiencing overload and ego-centred conflict patterns showed higher levels of psychological distress. These results highlight the importance of considering the structural dimensions of positive and negative relationships together to understand the lasting connection between family networks and the psychological health of individuals with mental illness.