Background <p>Previous studies have examined the correlation between social support and adolescent mental health, as well as the mediating role of self-efficacy in this association, primarily using cross-sectional data. However, few have employed longitudinal designs, considered both positive and negative aspects of mental health, or focused specifically on adolescents from financially strained families. This study aimed to investigate the mediating role of self-efficacy in the relationship between family support and both positive and negative dimensions of mental health among Taiwanese adolescents and youths from low-income families.</p> Methods <p>Using two-wave longitudinal panel data, the study included 1,792 economically disadvantaged adolescents and youths across 23 counties and cities in Taiwan. Life satisfaction and psychological distress were used to indicate positive and negative dimensions of mental health, respectively. A half-longitudinal mediation analysis was conducted.</p> Results <p>Family support at T1 was positively associated with self-efficacy and life satisfaction but not psychological distress at T2. Self-efficacy at T1 was significantly related to life satisfaction but not psychological distress at T2. The half-longitudinal mediation analysis further supported the mediating role of self-efficacy in the association between family support and life satisfaction.</p> Conclusions <p>Family support was predictive of positive, but not negative, aspects of mental health. In addition, self-efficacy served as a mediator in the link between family support and positive mental health. These findings underscore the importance of policies and interventions that strengthen both family support and self-efficacy while also highlighting the need for differentiated approaches to address the positive and negative dimensions of mental health among adolescents and youths from low-income families in Taiwan.</p>