This systematic review and meta-analysis explores the effectiveness of teacher interventions supporting children with externalizing behaviors based on teacher and child outcomes. A systematic search was conducted using 5 electronic databases. From 5714 papers, 31 papers that included interventions delivered directly to teachers and aimed to benefit either teachers and/or children with externalizing behaviors were included. The review focused on qualified teachers working with children aged 2–13. The results of the current meta-analysis revealed a positive effect of teacher intervention on teacher and child outcomes, including the increased use of teacher-appropriate strategies, as well as significant and moderate improvements in teacher–child closeness, and small reductions in teacher–child conflict. For child outcomes, the interventions reduced externalizing behavior problems and ADHD symptoms and enhanced prosocial behavior. Only one fully blinded analysis for conduct problems was possible and revealed a moderate but significant reduction in favor of intervention. These findings provide evidence to support the role of teacher interventions for both teachers and children with externalizing behaviors. Future research should include more PBLIND measurements so that MPROX findings can be confirmed. More research should be done to evaluate the influence of teacher interventions on teachers’ well-being.