Background <p>This study aimed to investigate the prevalence and impact of emotional and behavioural dysregulation (EBD) symptoms in childhood/early adolescence reported in primary care among young adults with severe mental illness.</p> Methods <p>Data were from electronic health records from general practices registered on the Clinical Practice Research datalink (CPRD) from 1987 to 2016 on young adults (aged 18 to 24 years) with mental illness who also had at least one primary care consultation for any reason while they were 8 to 14 years old. Using logistic regression, we examined the association of several EBD symptoms at age 8–14 years with severe mental illness rather than common mental illness in early adulthood.</p> Results <p>161,654 young adults aged 18–24 with mental disorder were included in the analysis (63.9% females; 9.4% classified severe). 19.0% had consultations for EBD symptoms at age 8–14. EBD symptoms in childhood/early adolescence increased the likelihood of later severe rather than common mental illness (OR = 1.30, CI: 1.20–1.41). Among symptoms which predicted a higher likelihood of severe mental illness were self-harm (OR = 3.05, CI: 2.06–4.73), suicidal ideation (OR = 2.34 CI: 1.15–4.76), behavioural problems (1.43, CI: 1.22–1.67) and mood symptoms (OR = 1.39, CI: 1.08–1.80). Anxiety, sleep difficulties, eating symptoms and hyperactive behaviours in childhood were not significantly associated with severe mental illness in young adults.</p> Conclusion <p>Primary care consultations for EBD symptoms, particularly self-harm, suicidal ideation, behavioural problems and mood symptoms, at age 8–14 years are associated with poorer mental health in young adulthood.</p>