OBJECTIVE: To describe the health and well-being of children and young people (CYP) seeking asylum subjected to Australia's immigration policy of indefinite mandatory detention on Nauru. DESIGN: Cross-sectional analysis of a cohort of CYP seeking asylum. SETTING: Australian paediatric clinicians from 10 health services completed detailed health assessments around the time of transfer from Nauru, mostly to Australia. PARTICIPANTS: Sixty-two CYP who were ≤18 years on entry into offshore immigration detention on Nauru between 2013 and 2019. Mean age at health assessment was 9 years. MAIN MEASURES: Health outcomes were categorised as physical, mental or neurodevelopmental concerns/conditions. Risk and protective factor data were collected using the adverse childhood experiences and refugee-specific adverse childhood experiences tools. RESULTS: Over half of the CYP (n=32, 52%) were held on Nauru for ≥4 years. The vast majority of CYP had physical health (n=55, 89%) and mental health (n=49, 79%) concerns including self-harm or suicidal ideation/attempt (n=28, 45%). Mental health concerns were more likely in CYP who were school-aged (p=0.001), had been held on Nauru for ≥1 year (p=0.01); originated from the Eastern Mediterranean region (p<0.05); witnessed trauma (p<0.05) or had exposure to ≥4 refugee-specific adverse childhood experiences (p<0.05). Neurodevelopmental concerns were seen in eight children (13%). CONCLUSIONS: This study highlights the almost universal physical and mental health difficulties in a sample of CYP who experienced forced migration and were subjected to Australia's offshore immigration detention policy. Immigration detention in recipient countries, a known adverse childhood experience, may contribute to or exacerbate harmful outcomes in CYP seeking asylum.