Adolescents with intellectual disability have substantial health needs. This retrospective analysis of data from the Ask Study describes reasons for primary care encounters and the prevalence and incidence of chronic physical and mental conditions among a cohort of community-dwelling adolescents with intellectual disability. Participants attended secondary schools in southern Queensland, Australia. Primary care data were extracted from primary care records. Demographic and health information was collected using carer-completed questionnaires. Reasons for primary care encounters, disease prevalence at age 16 years, and disease incidence through adolescence were reported. Data were obtained for 432 adolescents with intellectual disability (median follow-up: 4.1 years). Skin problems (29.4 per 100 encounters) were the most common reason patients presented for primary care, followed by psychological and behavioural problems (14.4 per 100 encounters) and musculoskeletal problems (13.8 per 100 encounters). Conditions with the highest prevalence were autism spectrum disorder (18.6%) and asthma (18.1%). The prevalence of epilepsy, visual impairment, and cerebral palsy were 14.7, 11.1, and 8.0%, respectively. Gastroesophageal reflux had the highest incidence (9.4 cases per 1000 person-years). Adolescents with intellectual disability have significant healthcare needs, which general practitioners need to be aware of and address. Study findings should inform the development of training programs for general practitioners.