<p>Child and adolescent mental health services (CAMHS) provide a critical opportunity for early mental health interventions. Evidence is still scare, however, on the long-term mental health outcomes of CAMHS patients, including their subsequent adult mental health service use. In a total population birth cohort, we linked national Scottish birth, death, and health service records for individuals born 1991–1998 (<i>N</i> = 490,779). We calculated the proportion of adult mental health service use that occurred among former CAMHS patients, as well as their inpatient adult diagnoses, by a maximum age of 32. We also examined prognostic factors in CAMHS for subsequent adult mental health inpatient admission. In total, 8.14% (<i>n</i> = 9,970) of the cohort had a recorded CAMHS attendance. The cumulative risk of going on to attend adult mental health services was 40.46% for all individuals who attended CAMHS and 49.19% for individuals who attended CAMHS in adolescence. Within adult mental health services, 41.13% of all outpatient appointments (<i>k</i> = 167,277/406,665) and 46.43% (<i>k</i> = 317,292/683,402) of all adult inpatient bed days were for individuals who had formerly attended CAMHS. Almost half of all adult inpatient bed days and 41% of outpatient appointments in Scotland by age 32 are attributable to individuals who had previously attended CAMHS. These findings demonstrate the frequently long-term mental health burden on individuals who come to CAMHS and highlight the need to evaluate the effects of interventions in childhood and adolescence on long-term mental health and functional outcomes.</p>