Organizational research has predominantly adopted the classic dispositional perspective to understand the importance of personality traits in shaping work outcomes. However, the burgeoning literature in personality psychology has documented that personality traits, although relatively stable, are able to develop throughout one's whole adulthood. A crucial force driving adult personality development is transition into novel work roles. In this article, we introduce a dynamic, role-based perspective on the adaptive nature of personality during the transition from the role of employee to that of leader (i.e., leadership emergence). We argue that during such role transitions, individuals will experience increases in job role demands, a crucial manifestation of role expectations, which in turn may foster growth in conscientiousness and emotional stability. We tested these hypotheses in two 3-wave longitudinal studies using a quasi-experimental design. We compared the personality development of 2 groups of individuals (1 group promoted from employees into leadership roles and the other remaining as employees over time), matched via the propensity score matching approach. The convergent results of latent growth curve modeling from the 2 studies support our hypotheses regarding the relationship between becoming a leader and subsequent small, but substantial increases in conscientiousness over time and the mediating role of job role demands. The relationship between becoming a leader and change of emotional stability was not significant. This research showcases the prominence of examining and cultivating personality development for organizational research and practice. (PsycInfo Database Record (c) 2020 APA, all rights reserved).