The purpose of this study was to investigate whether differentiation of self mediates the relationship between parenting styles (i.e., care and overprotection) and the level of career decision-making among young adults. We separately measured the mother’s and the father’s parenting styles and five dimensions of differentiation of self that S. Je (1989) suggested. The data came from 387 young adults who were juniors and seniors at universities in Seoul and its surrounding cities. Using Baron and Kenny’s (1986) steps based on regression, we found that the relationships between both caring and overprotective parenting styles and young adults’ level of career decision-making were mediated by four dimensions of differentiation of self (i.e., intellectual functioning vs. emotional functioning, family projection process, emotional cutoff, and family regression) except for integration of self. The results were consistent for both the mother and the father. Bootstrapping tests showed that all of the mediation effects were statistically significant. This study contributes to the literature by showing that young adults’ level of career decision-making is not simply determined by inputs such as parenting styles but is also mediated by differentiation of self. The results of this study are meaningful in that both the mother’s and the father’s parenting styles could make a difference in young adults’ level of career decision-making through differentiation of self.