Drawing on a sample of 318 African American and 354 Latino urban, low-income families, we identify maternal monitoring knowledge trajectories and examine which trajectory predicts fewer late-adolescent externalizing problems and which family and neighborhood factors predict trajectories with positive implications for late-adolescent externalizing behaviors. The majority of adolescents in both groups perceived long-term high levels of maternal monitoring knowledge throughout adolescence—stably high for the African American sample and high for the Latino sample. Long-term high levels of knowledge predicted fewer general late-adolescent externalizing problems for both groups and fewer late-adolescent delinquent behaviors for the African American sample. Family routine and mother–adolescent trust predicted long-term high levels of knowledge for both groups. For the African American sample, family routine and neighborhood cohesion predicted stably high levels of knowledge via the mediation of mother–adolescent trust. We discuss implications for improving positive adolescent development and family environments for both groups.