Hundreds of millions of people suffer anxiety disorders globally, demonstrating need for scalable and effective interventions. Adverse childhood experiences contribute to this mental health burden. The stress-buffering hypothesis, which posits social factors moderate prior adversity and subsequent mental health outcomes, provides one theoretical avenue to consider observations that group-based microfinance programs improve social capital. We investigate associations between adverse childhood experiences, generalized anxiety among adults and social capital associated with participation in a group-based microfinance program in rural Kenya. Adult participants (n = 400 women) responded to standardized measures of childhood adversity in June 2018, group-affiliated social capital and generalized anxiety in June 2019. Cumulative adverse childhood experiences predicted higher anxiety, which was statistically moderated by the presence of group-affiliated interpersonal trust. This study is the first to find social capital associated with participation in a group-based microfinance program statistically moderates expected associations between adverse childhood experiences and adult generalized anxiety. Future study should be conducted using a cluster-randomized control design to further assess the potential of this intervention method to ameliorate associations between past adversity and current mental health. •Adverse childhood experiences contribute to the high global burden of adult anxiety.•Population-based interventions are required to mitigate the adult effects of ACEs.•Group-based microfinance programs increase social capital with under-explored benefits.•Social capital moderates ACEs and adult anxiety among rural Kenyans.•Research should explore of economic, social and mental health synergies.