Mental disorders have high prevalence for disability and almost 80% of the global burden occurs in low- and middle-income countries. The impacts of mental health conditions can affect many sectors of society and threaten peace, human rights and development. However, international law jurisprudence has not sufficiently developed to guide mental health governance. This paper reviews the international legal protections for people who experience mental health conditions, including mental disorders and psychosocial disabilities. It focuses on State application of legal instruments in particularly vulnerable contexts, namely, least developed countries and situations of armed conflict. It argues that relying on existing treaties and soft-law instruments from the health and human rights angles is inadequate, and the Convention on the Rights of Persons with Disabilities is not the right fit. New hard- and soft-law instruments are urgently needed to meet positive obligations and safeguard rights in these vulnerable contexts. Some suggestions for the contents of future instruments are made.