Public mental health involves a population approach to mental health, and includes treatment of mental disorder, prevention of associated impacts, prevention of mental disorder and promotion of mental well-being, including for those people recovering from mental disorder. Such interventions can result in a broad range of impacts and associated economic savings even in the short term. However, even in high-income countries only a minority of people with mental disorder receive any treatment, while provision is far less in low- and middle-income countries. Coverage of interventions to prevent mental disorder and promote mental well-being is far less even in high-income countries, despite such interventions being required for sustainable reduction in the burden of mental disorder. This implementation gap results in a broad set of impacts and associated economic costs. Mental health needs assessments represent an important framework and mechanism to address this implementation gap - in low- and middle-income as well as high-income countries. Training and support to perform mental health needs assessments is important, as is the use of information derived from such assessments to more effectively advocate for the required level of resources to address the implementation gap. Such a public health approach to mental health represents an opportunity for psychiatrists to advocate more effectively for resources at both the local and national level. This can improve the coverage and outcomes of a range of public mental health interventions that result in broad impacts and associated economic savings, which can be estimated.