Abstract: Objective: The study examines the impact of ADHD on cognitive and functional abilities in post-9/11 Veterans with PTSD. Method: 313 Veterans from the Translational Research Center for TBI and Stress Disorders at VA Boston with PTSD only (44.73%), with ADHD only (3.51%), with both PTSD+ADHD (8.63%), and without either condition (43.13%) were evaluated. PTSD was assessed by the Clinician-Administered PTSD Scale (CAPS-IV). ADHD was assessed by the Wender Utah Rating Scale for ADHD (WURS ≥46). WHO Assessment Schedule (WHODAS) measured general functioning. Linear regression models analyzed differences in neuropsychological (Digit Span, Stroop, FAS, Trails, Coding, California Verbal Learning Test, and the Brief Visuospatial Memory Test) and general functioning among the four groups.Results: Veterans with PTSD only performed significantly worse than all other groups on Stroop–Color Naming (β = −0.77, p = 0.022), and FAS–Category Fluency (β = −1.10, p = 0.014). Veterans with both ADHD+PTSD performed significantly slower than all other groups on Trails–Number Sequencing (β = −1.65, p = 0.003). Veterans with PTSD only and with PTSD+ADHD reported significantly worse functional status (β’s = 13.56–24.74, p’s < 0.0001), mood (β’s = 7.87–17.30, p’s < 0.0001), sleep quality (β’s = 5.06–6.73, p’s < 0.0001), and neurobehavioral symptoms (β’s = 16.37–25.36, p’s < 0.0001) compared to other groups. Conclusion: These findings suggest that PTSD has a larger impact on cognitive performance, specifically processing speed, and functioning than ADHD alone. However, when combined, ADHD compounds the effects of PTSD, resulting in worse functioning across multiple domains compared to Veterans who have only ADHD or PTSD.