Background: About 25% of the worldwide population suffers from mental, neurological and substance use disorders but unfortunately, up to 75% of affected persons do not have access to the treatment they need. Data on the magnitude of the mental health problem in Kenya is scarce. The objectives of this study were to establish the prevalence and the socio-demographic factors associated with mental and substance use disorders in Kosirai division, Nandi County, Western Kenya. Methods: This was a cross sectional descriptive study in which participants were selected by simple random sampling. The sampling frame was obtained from a data base of the population in the study area developed during door-to-door testing and counseling exercises for HIV/AIDS. Four hundred and twenty consenting adults were interviewed by psychologists using the Mini International Neuropsychiatric Interview Version 7 for Diagnostic and Statistical Manual 5th Edition and a researcher-designed social demographic questionnaire. Results: One hundred and ninety one (45%) of the participants had a lifetime diagnosis of at least one of the mental disorders. Of these, 66 (15.7%) had anxiety disorder, 53 (12.3%) had major depressive disorder; 49 (11.7%) had alcohol and substance use disorder. 32 (7.6%) had experienced a psychotic episode and 69 (16.4%) had a life-time suicidal attempt. Only 7 (1.7%) had ever been diagnosed with a mental illness. Having a mental condition was associated with age less than 60 years and having a medical condition. Conclusion: A large proportion of the community has had a mental disorder in their lifetime and most of these conditions are undiagnosed and therefore not treated. These findings indicate a need for strategies that will promote diagnosis and treatment of community members with psychiatric disorders. In order to screen more people for mental illness, we recommend further research to evaluate a strategy similar to the home based counseling and testing for HIV and the use of simple screening tools.