The purpose of this longitudinal study is to examine the effects of personal factors and contextual determinants on the career maturity change of Korean adolescents over a 5-year period. This study used data from the Korea Youth Panel Survey which was administered to 3,449 junior high students from Grades 8 to 12, starting in 2003. A linear mixed-effects regression was used to test the study model. The results showed the levels of Korean adolescents' career maturity were significantly influenced by personal predictors (gender, work values, career efficacy, self-efficacy, career development activities, school achievement, sex-role stereotyping, pressure for academic achievement, and part-time experiences) and contextual predictors (career conversations with parents, relationships with friends, and private education expense). Among career-related variables, only work values affected the slope of the career maturity growth curve. Implications for career counselors and educators are discussed.