Mental disorders (including substance use disorders, dementia, and self-harm) account for a substantial burden of disease and economic costs in low-income and middle-income countries (LMICs), yet they attract little funding. External resources are urgently needed but evidence on investments is scarce. This Health Policy paper uses 35 elite interviews and documentary analyses to examine how and why external organisations have invested in mental health in LMICs over the past three decades, and how this investment has changed over time. Four levels are examined: organisations, source countries, recipient countries, and global landscape. Organisations have invested in numerous internal and external activities. Among the various factors shaping organisational decisions, actors (ie, individuals and organisations concerned with mental health) were the most salient at all four levels. To increase external organisation investments in mental health in LMICs, organisational leadership and understanding are crucial, along with increased political support in source and recipient countries, and a stronger governance structure at the global level.