Few epidemiological studies presented 12-month and lifetime prevalence estimates for DSM-IV mental disorders in the adult general population by sex and age up to very old age. From 2007 to 2010, DSM-IV mental disorders were assessed with the DIA-X/M-CIDI among N = 2400 participants (aged 29–89 years) from the Study of Health in Pomerania, an epidemiological study based on a two-stage stratified cluster sample randomly drawn from the adult general population in northeastern Germany. 36.3% of the sample was affected by any 12-month and 54.8% by any lifetime mental disorder. The most frequent diagnostic groups were anxiety (12-month: 14.8%, lifetime: 23.4%), substance use (12-month: 14.5%, lifetime: 25.0%), somatoform (12-month: 12.9%, lifetime: 20.4%) and depressive (12-month: 7.3%, lifetime: 18.6%) disorders. Except for substance use (higher prevalence in men) and bipolar disorders (comparable prevalence in men and women), higher 12-month and lifetime prevalence estimates were found in women vs. men. Moreover, lower 12-month and lifetime prevalence estimates were found in older (aged 60–74 or 75–89 years) vs. younger (aged 29–44 or 45–59 years) age groups. 22.6% (men: 21.1%, women: 23.9%) of those affected by any 12-month disorder met criteria for two and 13.6% (men: 9.6%, women: 16.9%) for three or more 12-month diagnoses. Similarly, 26.4% (men: 25.7%, women: 26.9%) of those affected by any lifetime disorder met criteria for two and 22.7% (men: 19.6%, women: 25.2%) for three or more lifetime diagnoses. Our findings demonstrate the frequency of mental disorders in northeastern Germany and emphasize the need for continued prevention and intervention efforts.