Youth mental health has declined over the past two decades, driving increased demand for child and adolescent psychiatry services. Little is known, however, about how diagnoses and related resource allocation is changing. Our aim was to analyse trends in child and adolescent psychiatry diagnoses and service use over time. We examined administrative health record data for all child and adolescent psychiatry service appointments between 2013 and 2023 for children (aged 0–12 years) and adolescents (aged 13–17 years) living in Finland. Stratifying by diagnosis, annual rates were calculated for inpatient and outpatient service use. Negative binomial regression was used to assess trends over time across diagnostic categories. The number of individuals attending psychiatric services rose by 137.5% in children and 112.4% in adolescents. Total number of appointments increased by 62.3% in children and 50.3% in adolescents. The mean number of appointments per individual declined from 7.8 to 5.3 for children and 7.9 to 5.6 for adolescents. Most diagnostic categories had increased presentations, with the largest absolute increases for attention deficit hyperactivity disorder (ADHD) in children (+ 59,138 appointments, average annual percentage change [AAPC] 15.28%) depressive disorders in adolescents (+ 30,213 appointments, AAPC 5.16%), and anxiety disorders in adolescents (+ 28,027 appointments, AAPC 6.04%). Number of appointments for people with schizophrenia-spectrum and bipolar disorders declined despite stable case numbers. Overall, CAMHS attendance has dramatically increased with substantial variation by diagnosis. Declining clinical time allocated to individuals with schizophrenia-spectrum and bipolar disorders raises concerns about equitable service access for young people with severe mental illnesses.