To better prepare students for active and thoughtful participation in a democratic society, civic education should foster an array of civic competencies. Cultivating student civic agency—an under-studied civic competency—is of particular importance to equip students to authentically use their voice in their communities. But what does it look like to foster student civic agency in a classroom setting? This article leverages a social and emotional learning (SEL) framework to uncover the active curricular ingredients and educational mechanisms through which a student-led civics curriculum has fostered civic agency. In this multi-site case study, we analyze the implementation of an eighth grade student-led civics curriculum in a northeastern U.S. state. We outline how implicit SEL processes in these classrooms, stemming from a pedagogical focus on “changemakers,” supported development of civic agency. We found that the changemakers theme facilitated student exploration of their own civic identity and values, while prompting students to critically examine political power structures. Notably, the theme also helped students to develop a narrative of self as changemakers and to envision themselves actively engaged in civic spaces. This study adds insights to growing evidence on the multi-faceted benefits that civically-oriented SEL may offer to preparing young people to engage in democracy.