A few epidemiological studies have examined personality disorders (PDs) among children and adolescents in secondary mental health services. This study aims to describe the prevalence and incidence of PDs among children and adolescents who have attended Danish child and adolescent psychiatric services (CAPS). Using register-based data, we studied all patients under the age of 18 years who were admitted to in- and outpatient CAPS (N = 115,121) in Denmark from 2007 to 2017. A total of 4952 patients were diagnosed with a PD during the study period. The mean prevalence was 859 patients per year, and the mean incidence was 274 patients per year, including an increased incidence and prevalence of borderline, anxious, and unspecified PDs over the decade. The number of patients diagnosed with PDs increased from 700 to 851 per year, but the proportion of patients with PDs compared to all psychiatric diagnoses decreased from 4.2% to 2.8% over the study period. The PD population had an older age (14.8 years vs. 11.3 years; p < 0.001), a higher likelihood of being female (74% vs. 44%; p < 0.001), and four times more contacts with the psychiatric emergency departments than other patients with a psychiatric diagnosis. Future studies should focus on (a) implementing further epidemiological studies in different countries; (b) tracking diagnostic practices to facilitate comparisons and provide feedback for training clinicians and raising awareness; and (c) estimating trajectories of PDs, including costs within the CAPS, to facilitate informed decision-making regarding the future organization and provision of services to these children, adolescents, and their families.