Objectives: To determine if levels of neighbourhood poverty and neighbourhood antisocial behaviour modify associations between household poverty and child and youth mental health problems. Methods: Data come from the 2014 Ontario Child Health Study—a provincially representative survey of 6537 families with 10,802 four- to 17-year-olds. Multivariate multilevel modelling was used to test if neighbourhood poverty and antisocial behaviour interact with household poverty to modify associations with children’s externalizing and internalizing problems based on parent assessments of children (4- to 17-year-olds) and self-assessments of youth (12- to 17-year-olds). Results: Based on parent assessments, neighbourhood poverty, and antisocial behaviour modified associations between household poverty and children’s mental health problems. Among children living in households below the poverty line, levels of mental health problems were 1) lower when living in neighbourhoods with higher concentrations of poverty and 2) higher when living in neighbourhoods with more antisocial behaviour. These associations were stronger for externalizing versus internalizing problems when conditional on antisocial behaviour and generalized only to youth-assessed externalizing problems. Conclusion: The lower levels of externalizing problems reported among children living in poor households in low-income neighbourhoods identify potential challenges with integrating poorer households into more affluent neighbourhoods. More important, children living in poor households located in neighbourhoods exhibiting more antisocial behaviour are at dramatically higher risk for mental health problems. Reducing levels of neighbourhood antisocial behaviour could have large mental health benefits, particularly among poor children.