Certain population subgroups are at higher risk of mental disorders because of greater exposure and vulnerability to unfavourable social, economic, and environmental circumstances, interrelated with gender. Disadvantage starts before birth and accumulates throughout life. A significant body of work now exists that emphasizes the need for a life course approach to understanding and tackling mental and physical health inequalities. This approach takes into account the differential experience and impact of social determinants throughout life. A life course approach proposes actions to improve the conditions in which people are born, grow, live, work, and age. Actions that prevent mental disorders and promote mental health are an essential part of efforts to improve the health of the world’s population and to reduce health inequities. There is firm consensus on known protective and risk factors for mental disorders. In addition, a growing body of evidence exists, not only from high-income countries but growing in low- and middle-income countries, that shows effective actions can be successfully implemented in countries at all stages of development.