Half of mental disorders have their first onset before adulthood when the presence of a disorder may be particularly disruptive to developmental milestones. Retrospective prevalence estimates have been shown to underestimate the burden of mental illness and scarce data are available on the incidence of disorders throughout the adolescent period, especially in developing countries. Thus, the objective was to determine the incidence of mental disorders in an 8-year period from adolescence to young adulthood, onset of service use and their predictors in a Mexican cohort. 1071 respondents from a representative two-wave panel sample participated in the Mexican Adolescent Mental Health Survey in 2005 and in the follow-up survey in 2013. Disorders were evaluated with the World Mental Health Composite International Diagnostic Interview. 37.9% experienced the onset of a psychiatric disorder and 28.4% sought services for the first time. Substance use disorders had the greatest incidence, followed by mood and behavior disorders, anxiety disorders and lastly eating disorders. Sex, age, school dropout, childhood adversities and prior mental disorders predicted the onset of new disorders. Being female, having more educated parents and most classes of disorder predicted first time service use. These findings contribute to a paradigm shift in conceptions of mental disorder similar to how we think of common physical afflictions as near universal experiences across the life course, but less frequent at any given moment. Adolescents are particularly vulnerable. Therefore, public health policy should focus on early universal promotion of positive mental health and structural determinants of mental health.