Parental incarceration (PI) is negatively associated with emotional, educational, and psychological child outcomes. However, few studies explore potential mechanisms through which these outcomes are transmitted or the means by which prosocial outcomes might develop. This study used data from two waves of a population cohort study of children aged 9 years and followed up aged 13 years living in Ireland. Children and parents (N = 8,568) completed measures of PI, primary caregiver (PCG) depression, PCG-child relationship quality, and child behavioral adjustment. We then conducted a secondary analysis on this national longitudinal study of children in Ireland. Using sequential mediation models, we observed a mediated indirect effect of PI on prosocial outcomes via PCG depression and PCG-child relationship quality. PI at age 9 was associated with increased difficulties and reduced prosocial behavior at age 13. Additionally, PI at age 9 affected PCG depression and the PCG-child relationship quality. Additionally, child prosocial outcomes, and emotional and behavioral difficulties were less apparent where PI had a weaker effect on PCG depression and the quality of PCG-child relationship. Supports that can mitigate the impact of PI for vulnerable caregivers and children are discussed.