This study examined the mediating role of maternal unsupportive parenting in explaining associations between family instability and children's externalizing symptoms during the transition to formal schooling in early childhood. Participants included 243 preschool children (M age = 4.60 years) and their parents. Findings from cross-lagged autoregressive models conducted with multimethod (survey and observations), multi-informant (parent, teacher, and observer), longitudinal (three annual waves of data collection) data indicated that experiences with heightened family instability predicted decreases in supportive parenting, which in turn predicted increases in children's externalizing symptoms. Analyses also revealed a bidirectional association between parenting and family instability over time, such that higher levels of instability predicted decreases in supportive parenting, which in turn predicted increases in family instability.