Background The prevention of mental disorders and promotion of mental health and well-being are growing fields. Whether mental health promotion and prevention interventions provide value for money in children, adolescents, adults, and older adults is unclear. The aim of the current study is to update 2 existing reviews of cost-effectiveness studies in this field in order to determine whether such interventions are cost-effective. Methods and findings Electronic databases (including MEDLINE, PsycINFO, CINAHL, and EconLit through EBSCO and Embase) were searched for published cost-effectiveness studies of prevention of mental disorders and promotion of mental health and well-being from 2008 to 2020. The quality of studies was assessed using the Quality of Health Economic Studies Instrument (QHES). The protocol was registered with PROSPERO (# CRD42019127778). The primary outcomes were incremental cost-effectiveness ratio (ICER) or return on investment (ROI) ratio across all studies. A total of 65 studies met the inclusion criteria of a full economic evaluation, of which, 23 targeted children and adolescents, 35 targeted adults, while the remaining targeted older adults. A large number of studies focused on prevention of depression and/or anxiety disorders, followed by promotion of mental health and well-being and other mental disorders. Although there was high heterogeneity in terms of the design among included economic evaluations, most studies consistently found that interventions for mental health prevention and promotion were cost-effective or cost saving. The review found that targeted prevention was likely to be cost-effective compared to universal prevention. Screening plus psychological interventions (e.g., cognitive behavioural therapy [CBT]) at school were the most cost-effective interventions for prevention of mental disorders in children and adolescents, while parenting interventions and workplace interventions had good evidence in mental health promotion. There is inconclusive evidence for preventive interventions for mental disorders or mental health promotion in older adults. While studies were of general high quality, there was limited evidence available from low- and middle-income countries. The review was limited to studies where mental health was the primary outcome and may have missed general health promoting strategies that could also prevent mental disorder or promote mental health. Some ROI studies might not be included given that these studies are commonly published in grey literature rather than in the academic literature. Conclusions Our review found a significant growth of economic evaluations in prevention of mental disorders or promotion of mental health and well-being over the last 10 years. Although several interventions for mental health prevention and promotion provide good value for money, the varied quality as well as methodologies used in economic evaluations limit the generalisability of conclusions about cost-effectiveness. However, the finding that the majority of studies especially in children, adolescents, and adults demonstrated good value for money is promising. Research on cost-effectiveness in low-middle income settings is required. Trial registration PROSPERO registration number: CRD42019127778.