There is a growing body of evidence to suggest that parental social capital may be beneficial for the mental wellbeing of children. While the mechanisms underlying this association may not yet be clearly established, the strong association alone presents an argument for building parental social capital as a way of preventing and/or overcoming difficulties with their children’s mental health. However, in order to encourage the building of parental social capital, an understanding of the building process and what might contribute to its formation is required. This study explores the processes involved in building social capital amongst parents attending a peer-led parenting intervention group. Participants were 14 mothers who each attended the inner city-based group for parents of children aged 2-11 years. The mothers were interviewed about their experiences of attending the parenting group, and a critical realist grounded theory approach was used to develop a framework for understanding the process of building social capital within this context. Findings suggested that the following key processes were associated with building social capital: Personal Development, Making Connections, Feeling Safe, and Overcoming Differences. Each of these processes was facilitated by aspects of the intervention, as well as participant and facilitator attributes. These findings are described in detail, followed by suggestions for future research and implications for clinical intervention.