Family supportive supervisor behavior (FSSB) has emerged as a powerful resource to support the well-being and development of the employee by reducing the impact of potential stress caused by having to juggle work and family responsibilities. However, research to date offers limited insight into the antecedents and underlying processes that may trigger FSSB. Informed by prosocial motivation and work-family border theory, we investigate the extent to which family motivation is linked to FSSB, and how the latter mediates the relationship to subordinate in-role work performance. Furthermore, we examine the role of supervisor work-life balance as a contextual variable influencing our proposed associations. Using matched supervisor-subordinate data from an organization in Chile (196 subordinates, 75 supervisors), our findings revealed that supervisor family motivation relate to subordinate work performance via FSSB. Moreover, this positive association is moderated by supervisor work- life balance, such that family motivation is a particularly potent driver of FSSB and subsequent subordinate work performance when supervisors perceive they have a low work-life balance themselves. The implications for research and practice are discussed.