<p>Universal school-based interventions can improve student mental health outcomes, but their success depends on the moderating influence of implementation. This systematic review examined the factors that influence teachers’ implementation of universal school-based mental health interventions. Searches of a range of databases (e.g., PsychINFO) were conducted. <i>N</i> = 21,987 sources were identified, with <i>n</i> = 19 studies eligible for inclusion. Data were extracted and synthesized using Domitrovich et al. (ASMHP 1:6–28, 2008) Implementation Quality Framework. Seven studies were qualitative, eight were quantitative, and four used a mixed methods design. Quality assessment was undertaken using the Joanna Briggs Institute Critical Appraisal Checklists for Quantitative and Qualitative Research; quality was good overall, with 68% assessed as high-quality. Factors influencing implementation from all four levels of the Implementation Quality Framework (macro, school, individual, and programme) were identified, but there was a scarcity of research considering macro level factors. Teachers’ intervention perceptions and attitudes were more frequently researched. Findings suggest that, to optimise implementation outcomes, teacher training prior to implementation is beneficial. Specifically, training that promotes strategies to communicate with students and teach mental health content effectively can enhance comfort in the classroom and support responsiveness and engagement. It can also raise awareness around the importance of supporting the mental health of students, which has the potential to increase staff buy-in and produce more favourable intervention attitudes. Further research on relatively neglected factors, such as macro level factors and some school level factors, is needed to provide a comprehensive picture of the factors and conditions required to optimise implementation.</p>