Epidemiological evidence from population-based surveys suggest that the psychological well-being of adolescents has been severely affected by the COVID-19 pandemic itself, as well as by the safety measures implemented. The rationale of the study was to investigate the influence of the pandemic on psychiatric emergency service use, psychiatric admissions rates, emotional well-being, suicidality and self-harm behaviour in help-seeking children and adolescents. Retrospective cohort study of electronic patient records before and during the COVID-19 pandemic from the emergency out-patient facility of the department of child and adolescent psychiatry and psychotherapy of the Psychiatric University Hospital Zürich. The frequency of all emergency service contacts from 1 January 2019 to 31 June 2021 were described and the frequency of records compared in half-year intervals. Emotional well-being, behavioural problems, suicidality and self-harm were estimated based on the mental state examination notes of electronic patient records from the 1 March to the 30 April for the years 2019, 2020 and 2021. After an initial decline in emergency contacts at the beginning of the first lockdown, the use of the centralised emergency service increased during the subsequent months and has since stabilised at a significantly higher level than before the pandemic. Comparison of emergency contacts in the first half of 2019 with the first half of 2021 shows that the number of emergency phone contacts nearly doubled, emergency outpatient assessments increased by 40%, emergency bridging interventions increased by 230%, and inpatient admissions of minors to adult psychiatric inpatient units more than doubled because of lack of service capacity in child and adolescent psychiatry. The proportion of adolescents who reported suicidal ideation increased significantly by 15%, from 69% to 84%, and the proportion of adolescents who reported self-harm behaviour increased by 17%, from 31% to 48%. We found a significant increase in psychiatric service use, as well as in reported serious mental health symptoms such as suicidality and self-harm behaviour in help-seeking children and adolescents in the course of the pandemic. The child and adolescent psychiatric healthcare system is overburdened and down-referral of adolescents in need of ongoing therapy is becoming increasingly difficult. We recommend prioritising preventive and therapeutic measures to support the mental health of our children and adolescents alongside the somatic management of the COVID-19 pandemic.