Objectives: The study aimed to investigate the trajectory of mobile phone dependency and determine the longitudinal pathways between the trajectory of mobile phone dependency, social capital, and depression in early adolescents. Methods: This study used data from the 2nd to 5th wave of the 2,378 4th graders panel of the Korea Children and Youth Panel Survey 2010 (KCYPS 2010). Data were analyzed using Pearson’s correlations, Latent Growth Modeling (LGM), and Structural Equation Modeling (SEM) using SPSS 20.0 and AMOS 20.0. Results: First, from the fifth to eighth grade, the trajectory of mobile phone dependency of early adolescents increased over time, but the trend has gradually decreased. Second, in the fifth grade, the more psychological and emotional support one experienced from parents, peers, and teachers, the lower the intercept of mobile phone dependency, and the slower the trend of change in growth over time. Third, the mobile phone dependency in the fifth grade had a significant effect on depression in the eighth grade. Lastly, while social capital in early adolescence did not show a significant direct effect on depression, it had an indirect effect on depression through the intercept of mobile phone dependency. Conclusion: The results of this study suggest that supportive social relationships can be an important protective factor against mobile phone dependency and the emotional health of early adolescents. For prospective intervention in mobile phone dependency, it would be necessary to help early adolescents develop supportive relationships with parents, peers, and teachers and have positive emotional experiences with them.