Purpose There is substantial evidence suggesting high levels of mental health problems in unaccompanied and separated children (UASC). However, there is less focus on the first years of adulthood characterised by increased vulnerability and fear of expulsion. We aimed to describe the mental health of UASC on reaching adulthood, and how this was affected by uncertainty regarding their right-to-stay in France. Methods One hundred and ten youth aged 18–22 were recruited via child protection reception centres. We administered the Patient Health Questionnaire somatic (PHQ-15), anxiety (GAD-7) and depression (PHQ-9) modules, the Post-Traumatic Stress Disorder Checklist (PCL-5) and Connor–Davidson Resilience Scale (CD-RISC-10). Logistic regression analysis was performed with the dependent variable, a secure (versus uncertain) situation, defined as (1) detaining a residence permit and being in school, an apprenticeship or a salaried job, or (2) waiting for residence permit whilst occupying a salaried job. Results Of the sample, 19.3% reached criteria for a probable somatic disorder, 17.6% for anxiety and 28.7% for depression (score  ≥ 10); 41.8% were in an uncertain situation regarding their right-to-stay. Uncertainty was associated with higher anxiety ((OR per Interquartile range (95% CI), 1.77 (1.05–2.98)) and post-traumatic stress symptoms (2.05 (1.06–4.00)), lower resilience (0.50 (0.27–0.91)), and participants rating their anxiety ( p  = 0.02) and depressive symptoms ( p  = 0.003) as more severe since reaching adulthood. Conclusions Our findings suggest uncertainty regarding right-to-stay is associated with increased mental health symptoms, specifically anxiety and trauma-induced stress, thereby highlighting the vulnerability of UASC in their first years of adulthood. This calls for greater support during this transition period with regular symptom monitoring for timely psychological interventions.