The objectives of this study are to (1) quantify the joint association of inadequate housing (presence of pests, mold, and need for repairs) and neighbourhood antisocial behaviour (assault, victimization, theft, and break-ins) with children’s mental health problems (externalizing and internalizing); and (2) assess whether this joint association varies as a function of child age – the latter critical for understanding when interventions will have maximum impact. Using data from the 2014 Ontario Child Health Study and multilevel modeling, our findings indicate that (1) the magnitude of the association between inadequate housing and child mental health problems is stronger in neighbourhoods characterized by high levels of antisocial behaviour; and (2) the cross-level interaction between inadequate housing and neighbourhood antisocial behaviour is more pronounced among younger vs. older children for externalizing but not internalizing problems. Greater government intervention is required to ensure that families have access to adequate and affordable housing in neighbourhoods free from safety concerns. Policies and interventions must be implemented as early as possible in the life course to have maximum impact.