Civic engagement is a crucial component of adolescent development. Activities and relationships in the high school context both contribute to the roles of youth as unique political actors and shape their lifelong participation. Despite the dependence of civic engagement on sociocultural factors and its empowering benefits for youth of color, Latinx adolescents remain underrepresented in research literature. Increased understanding of the motivational beliefs and social processes that influence youth may inform policies and practices that both recognize and support the engagement of marginalized youth in our precarious political era. In the three studies of this dissertation, I applied expectancy-value motivation, critical consciousness, and social network analyses to examine psychological antecedents and social mechanisms that underlie youth participation in service and activism. The studies leveraged longitudinal survey data collected in spring 2019 and spring 2020 at a local high school that serves primarily low-income Latinx youth. In Chapter 1, I utilized OLS regressions and cluster analyses to demonstrate the conceptual utility of an expectancy-value model of youth civic motivation. I found that expectancies and values (1) differentially and interactively predicted service and activism, and (2) manifested heterogeneously among the students in the sample. In Chapter 2, I used Exponential Random Graph Models (ERGMs) and regressions to examine the relationships between characteristics of the high school friendship network and adolescents’ civic behaviors and beliefs. Service, activism, perceptions of inequities, civic values, and civic expectancies were differentially linked to adolescents’ tendency to be similar to their friends, as well as their popularity and the structure of their friend groups. In Chapter 3, I employed longitudinal social network analyses to investigate the functions of socialization and critical beliefs in civic behavior. Peer influence and friendship selection processes were present for service activities, but not activism, whereas perceptions of inequities positively predicted later activism, but not service. In Chapter 4, I summarized and synthesized findings across the three studies, then I discussed my personal reflections on the field of youth civic engagement. Together, my research advanced an expectancy-value model of civic motivation, demonstrated the utility of social network analyses for understanding youth civic engagement, clarified the link between perceptions of inequities and action, and broadened the representation of Latinx youth experiences in academic literature.