Objective: Previous studies in children with ADHD identified two partially-separable familial factors underlying cognitive dysfunction, but evidence in adolescents and adults is lacking. Here, we investigate the etiological structure of cognitive-neurophysiological impairments in ADHD in adolescents and young adults. Method: Factor analyses and multivariate familial models were run in 356 participants from ADHD and control sibling pairs aged 11-27 years on data on IQ, digit span forward (DSF) and backward (DSB), and cognitive-performance and event-related potential (ERP) measures from three cognitive tasks. Results: Three familial factors (cF1-3), showing substantial familial overlap with ADHD, captured the familial covariation of ADHD with nine cognitive-ERP measures. cF1 loaded on IQ, mean reaction time (MRT) and reaction-time variability (RTV); cF2 on DSF and DSB; and cF3 on number of errors and ERPs of inhibition and error processing. Conclusion: These results identify three partially-separable etiological pathways leading to cognitive-neurophysiological impairments in adolescent and adult ADHD.