The social determinants of health continue to impact health disparities among communities living along the U.S.-Mexico border. Because community health workers (CHWs) are recognized for promoting a variety of positive patient-centered health outcomes in their roles as educators and health system navigators, recent inquiry has focused on the role of the CHW in facilitating community-level changes through grassroots advocacy to impact the social determinants of health. Social capital theory, which posits that participation in groups has positive consequences for individuals and the community, is a useful lens through which CHW effectiveness in grassroots advocacy can be measured and replicated. Using quantitative and qualitative methods, this study investigated the social capital characteristics of fifteen CHWs working in border communities who were trained in grassroots advocacy. Participating CHWs reported high baseline levels of social capital, which was assessed using a social capital questionnaire. After one year of engagement in grassroots advocacy, participating CHWs reported statistically significant increases in one measure of bridging social capital (community engagement) and one measure of linking social capita (participation with political/civic leaders). Qualitative interviews with each CHW further explored the role of social capital in their grassroots advocacy, resulting in a social capital model elucidating six activities of CHW-driven grassroots advocacy. The mixed methods results provide measures for assessing CHWs' social capital in their traditional roles and as grassroots advocates, as well as a framework for understanding how CHWs use their social capital to take grassroots action addressing the social determinants of health.