Mental disorders in children and adolescents may follow different trajectories, such as remission, change of diagnosis, or addition of two or more comorbid diagnoses, showing a heterotypic pattern. This study aims to describe the main diagnostic trajectories across a broad range of mental disorder diagnostic categories, from childhood to adolescence and from adolescence to young adulthood in a clinical population. A prospective study was conducted among a clinical sample of children and adolescents who were aged 3–17 years at the face-to-face baseline interview. Electronic health records of these participants were reviewed 10 years later. The diagnostic stability over time was examined using the kappa coefficient, and factors associated with stability were explored using simple logistic regression. The study included a sample of 691 participants. The kappa coefficient for diagnostic stability across all diagnoses was 0.574 for the transition from childhood to adulthood, 0.614 from childhood to adolescence, and 0.733 from adolescence to adulthood. Neurodevelopmental diagnoses had the highest stability. Factors associated with higher diagnostic stability included family history of mental disorders, receiving psychopharmacological treatment, and symptom severity at baseline. We found a variable diagnostic stability across different diagnoses and age categories. The different life transitions represent complex periods that should not be overlooked from a clinical standpoint. An appropriate transition from child and adolescent mental health services to adult mental health services may have a positive impact on children and adolescents with mental disorders.