A cross-sectional population-based study was conducted to identify the prevalence of common mental disorders and verify the association with chronic non-communicable diseases (NCDs) and the self-reported number of chronic diseases. The Self-Reporting Questionnaire (SRQ-20) was applied in a multi-stage random sample of 1,276 adults aged 40 and older. Socio-demographic, behavioral, and health-related variables were also obtained using a structured questionnaire. Prevalence of common mental disorders was 30.2%. Lower schooling and social class and the 46-55-year age bracket were associated with psychiatric morbidity. Each chronic illness was independently associated with common mental disorders. However, a stronger association was found between common mental disorders and the total number of self-reported chronic conditions, with a prevalence ratio of 4.67 (95%CI: 3.19-6.83) for five or more self-reported NCDs. The current study emphasizes the importance of common mental disorders in chronically ill patients, particularly in those with more total chronic conditions.