Background: The Boston Circulatory Arrest Study (BCAS) is a longitudinal investigation of neurodevelopmental and mental health (MH) outcomes of newborns with d-TGA who underwent the arterial switch operation. Goal: We describe current and lifetime MH outcomes in this now adult cohort. Methods: We enrolled 88/149 surviving and eligible infant BCAS study participants (29±1.3 yrs; 73% male) between 2017 and 2021; 77 were evaluated prior to the COVID-19 pandemic. The Structured Clinical Interview for DSM-5 Disorders - Research Version, which includes modules exploring anxiety, mood, externalizing, trauma and stress-related, and substance use disorders, as well as suicidality, was administered and captured lifetime (any point in life) and current (within the last 1-12 months) diagnoses. Results: One participant was not interviewed and was referred for further care after endorsing current suicidal ideation. Of 87 interviewees, 62 (71%) met criteria for ≥1 lifetime psychiatric disorder, with 41% diagnosed with lifetime mood disorder and 25% with lifetime anxiety disorder. In addition, 37% noted lifetime substance use disorders, most commonly with alcohol use. Four (5%) had a lifetime history of post-traumatic stress disorder. For comparison, general US similar age population prevalence of lifetime mood, anxiety, and substance use disorders were 21-24%, 30-35% and 17-18% respectively. Forty (46%) participants met criteria for ≥1 current disorder, with 11% diagnosed with mood disorders and 16% with anxiety disorders and 22% with substance use disorders. Further, 31% of participants had a history of suicidal ideation, 10% of non-suicidal self-injury, and 6% of suicide attempt compared to 5%, 1% and 0.4% respectively in a similar age general US population. Conclusions: MH disorders are common in young adults with d-TGA, with almost three-quarters meeting criteria for a psychiatric disorder by age 35. Mood and anxiety disorders are particularly common, and almost one-third endorse a history of suicidal ideation. Substance use is also common and warrants inclusion in history-taking of adolescent and adult CHD patients. Ongoing surveillance to identify patients with MH disorders in order to initiate treatment is vital for young adults with CHD.