Background: One often-overlooked biological risk factor that may help explain sex-biased prevalence rates in psychopathology is sex hormones. Personality traits, which also show sex differences, may mediate relations between biological risk factors like hormones and childhood psychopathology such as ADHD (or, alternatively, be independent risk factors). Methods: Three hundred and twelve children/adolescents (178 boys, 134 girls) between the ages of 8 and 17 completed a comprehensive, multistage, clinical diagnostic procedure; 168 children were diagnosed with ADHD and 144 were classified as non-ADHD comparison controls. Primary caregivers completed the California Q-sort in order to provide a measure of conscientiousness. Finger-length ratios (specifically right 2D:4D) served as a proxy of prenatal testosterone exposure (relative to estrogen). Results: Lower levels of conscientiousness statistically mediated the relationship between more masculine right 2D:4D (i.e., increased prenatal testosterone exposure) and increased ADHD inattentive symptoms. Conclusion: More masculinized finger-length ratios show associations with ADHD symptoms, possibly acting through the trait mechanism of conscientiousness.