Abstract Background A large mental health treatment gap exists among conflict-affected populations, and Syrian refugees specifically. Promising brief psychological interventions for conflict-affected populations exist such as the World Health Organization’s Problem Management Plus (PM+) and the Early Adolescent Skills for Emotions (EASE) intervention, however, there is limited practical guidance for countries of how these interventions can be taken to scale. The aim of this study was to unpack pathways for scaling up PM+ and EASE for Syrian refugees. Methods We conducted three separate Theory of Change (ToC) workshops in Turkey, the Netherlands, and Lebanon in which PM+ and EASE are implemented for Syrian refugees. ToC is a participatory planning process involving key stakeholders, and aims to understand a process of change by mapping out intermediate and long-term outcomes on a causal pathway. 15–24 stakeholders were invited per country, and they participated in a one-day interactive ToC workshop on scaling up. Results A cross-country ToC map for scale up brief psychological interventions was developed which was based on three country-specific ToC maps. Two distinct causal pathways for scale up were identified (a policy and financing pathway, and a health services pathway) which are interdependent on each other. A list of key assumptions and interventions which may hamper or facilitate the scaling up process were established. Conclusion ToC is a useful tool to help unpack the complexity of scaling up. Our approach highlights that scaling up brief psychological interventions for refugees builds on structural changes and reforms in policy and in health systems. Both horizontal and vertical scale up approaches are required to achieve sustainability. This paper provides the first theory-driven map of causal pathways to help support the scaling-up of evidence-based brief psychological interventions for refugees and populations in global mental health more broadly.