Background Neurodevelopmental impairments are common in survivors of complex congenital heart defects (CHD). We report neuropsychological and brain imaging assessments in adults operated for isolated septal defects. Methods and Results Patients (mean age 25.6 yrs) who underwent childhood surgery for isolated atrial septal defect (n=34) or ventricular septal defect (n=32), and healthy matched peers (n=40), underwent a standard battery of neuropsychological tests and a 3.0T brain magnetic resonance imaging scan. Patient intelligence was affected with lower scores on Full-Scale intelligence quotient (P<0.001), Verbal Comprehension (P<0.001), Perceptual Reasoning (P=0.007), and Working Memory (P<0.001) compared with controls. Also, the CHD group had poorer visuospatial abilities (Immediate Recall, P=0.033; Delayed Recall, P=0.018), verbal memory (Trial 1, P=0.015; Total Learning, P<0.001; Delayed Recall, P=0.007), executive function (Executive Composite Score, P<0.001), and social recognition (Reading the Mind in the Eyes Test, P=0.002) compared with controls. Self-reported levels of executive dysfunction, attention deficits and hyperactivity behavior, and social cognition dysfunction were higher in the CHD group compared with population means and controls. We found similar global and regional morphometric brain volumes and a similar frequency of brain magnetic resonance imaging abnormalities in the 2 groups. The CHD group had a high occurrence of psychiatric disease and a larger need for special teaching during school age. Conclusions Children operated for simple CHD demonstrate poorer neurodevelopmental outcomes in adulthood when compared with healthy controls and expected population means. REGISTRATION URL: https://www.clini​caltr​ials.gov. Unique identifier: NCT03871881.