Extant research has identified that employee political skill exerts a positive influence on the formation of high-quality leader-member exchange (LMX), thereby impacting career outcomes. However, the boundary conditions that moderate this relationship, especially from an integrated career capital and social exchange perspective, remain underexplored. Grounded in these two theoretical perspectives, we argue that task performance, as human capital, enables political skill, conceptualized as social capability or relational competence, to translate into high-quality LMX as realized relational social capital. Specifically, we propose that task performance strengthens the political skill-LMX link, which in turn enhances promotability. Analyses of 200 supervisor-subordinate dyads support our hypotheses. These results yield substantive implications for organizational behavior theory and managerial practice.