National occupational health and safety (OHS) policy (e.g., legislation) underpins worker health protection and is imperative for healthy and safe working populations. In the interest of bolstering mental health through decent work, this study undertakes a global analysis of OHS policy for worker mental health and develops and validates a short tool quantifying national policy approaches—the National Policy Index (NPI, for worker mental health). Data were collected across 45 countries from 164 global experts (and/or expert groups) to capture policy presence, priority action areas, and drivers and barriers surrounding policy implementation. Analysis revealed top global psychosocial concerns are harassment, mobbing or bullying, work overload, discrimination, and poor work-life balance. Policy priorities are harassment, mobbing or bullying, discrimination, and physical violence. The psychosocial hazards/risks that are most addressed in policies or regulated are physical violence, discrimination, harassment, mobbing or bullying. The main driver for managing hazards is workplace senior management support and having specific national regulations, and the main barrier is poor resource availability. Further, the NPI was developed through exploratory factor analysis and validated through significant correlation with a national policy audit and to the 2019 European Survey of Enterprises on New and Emerging Risks data, which reports enterprise level psychosocial safety climate (PSC, organisational policies, practices, and procedures for stress prevention). The correlation between the NPI and enterprise-level PSC highlights the critical role of national policy in protecting worker population mental health. Yet above and beyond national policy, national union density also related to enterprise PSC indicating that social action is also imperative. Findings suggest that global mental health can be reinforced via decent work outlined in national policy approaches, particularly legislation, as well as via senior management support, and collective approaches such as union action.