This study sought to understand how young adults (age 18-25) with histories of mental health disorders are coping with disrupted transitions to adulthood during the COVID-19 pandemic. A cross-sectional web survey was conducted in March-June 2021 of 967 US young adults with pre-pandemic psychiatric disability to assess their current psychiatric status, interrupted transitions, and associations with social determinants including income, community participation, and social context. Mental health was assessed with the Patient Health Questionnaire (PHQ-9), Generalized Anxiety Disorder Scale (GAD-7), and PTSD Checklist-Civilian Version. Social determinants were identified with the Epidemic-Pandemic Impacts Inventory. Interrupted transitions were measured with the Young Adult Disrupted Transitions Assessment. Multivariable logistic regression models predicted four types of transition disruptions and associations with current mental health, social determinants, and demographic factors. Disruptions were reported by 81.1% including interrupted education completion (38.3%), employment careers (37.6%), residential independence (27.7%), and intimate partner relationships (22.9%). Many screened positive for major depressive disorder (81.7%), PTSD (85.5%), or GAD (58.6%). Disruption in establishing intimate partner relationships was associated with depression, anxiety, and PTSD. Interrupted residential independence was associated with anxiety. Interrupted education completion was associated with PTSD. Interrupted employment was associated with anxiety. Social determinants significant in these models included social connections, community participation, income, and racial/ethnic identification. Results illuminate ways that current mental health and social determinants affect transition interruptions during the pandemic. Findings suggest the need for interdisciplinary approaches, integrated models of care, and assistance accessing treatment, rehabilitation, and community support services from adult service systems.