This study examined the mediating role of ego identity in the relationship between mobile phone dependency and school adjustment among Korean youth. The topic is timely as adolescent problem behaviors have emerged with the increase in mobile phone use and dependency. Waves 1 and 3 of the Korean Children and Youth Panel Survey(N= 1,906) were examined using Pearson correlation analysis and mediation analysis while controlling for youth demographic and socioeconomic characteristics. There was a strong correlation among mobile phone dependency, school adjustment, and ego identity. Additionally, mobile phone dependency(7th grade), mediated by ego identity(9th grade), was inversely associated with school adjustment(9th grade). The indirect effect was significant when controlling for mobile phone dependency(9th grade), gender, parental education, and household income. Results from this study may provide implications for designing early preventive programs against excessive mobile phone use and dependency among Korean adolescents.