ObjectivesMental disorders typically emerge during adolescence and young adulthood and put young people at risk for prolonged socioeconomic difficulties. This study describes the longitudinal course of social and occupational functioning of young people attending primary care-based, early intervention services.DesignA longitudinal study of young people receiving mental healthcare.SettingData were collected between January 2005 and August 2017 from a designated primary care-based mental health service.Participants554 young people (54% women) aged 12–32 years.MeasuresA systematic medical file audit collected clinical and functional information at predetermined time intervals (ie, 3 months to 5+ years) using a clinical pro forma. Group-based trajectory modelling (GBTM) was used to identify distinct trajectories of social and occupational functioning over time (median number of observations per person=4; median follow-up time=23 months).ResultsBetween first clinical contact and time last seen, 15% of young people had reliably deteriorated, 23% improved and 62% did not demonstrate substantive change in function. Of the whole cohort, 69% had functional scores less than 70 at time last seen, indicative of ongoing and substantive impairment. GBTM identified six distinct functional trajectories whereby over 60% had moderate-to-serious functional impairment at entry and remained chronically impaired over time; 7% entered with serious impairment and deteriorated further; a quarter were mildly impaired at entry and functionally recovered and only a small minority (4%) presented with serious impairments and functionally improved over time. Not being in education, employment or training, previous hospitalisation and a younger age at baseline emerged as significant predictors of these functional trajectories.ConclusionYoung people with emerging mental disorders have significant functional impairment at presentation for care, and for the majority, it persists over the course of clinical care. In addition to providing clinical care earlier in the course of illness, these data suggest that more sophisticated and more intensive individual-level and organisational strategies may be required to achieve significant and sustained functional improvements.