This prospective longitudinal study examined how maternal prenatal substance use disorder (SUD) and early mother-infant interaction quality are associated with child social cognition (emotion recognition and mentalization) at school age. A sample of 52 poly-substance-using mothers receiving early interventions and 50 non-users, along with their children, was followed from pregnancy to school age. First-year mother-infant interaction quality was measured with EA scales. At school age, child facial emotion recognition was measured with DANVA and mentalization with LEAS-C. SUD group children did not differ from comparison children in social cognition, but higher severity of maternal prenatal addiction predicted emotion recognition problems. High early mother-infant interaction quality predicted better emotion recognition and mentalization, and mother-infant interaction quality mediated the effect of prenatal SUD on emotion recognition. The results highlight the need for early treatments targeting both parenting and addiction, as well as long-term developmental support for these children.