Background: In addition to symptoms of neurodevelopmental disorders, functional impairment is crucial to the determination of clinical significance. The aim of this study was to examine partial validity and usefulness of the Strengths and Difficulties Questionnaire's (SDQ) impact supplement (SDQ impact) in measuring functional impairment in children and adolescents diagnosed with attention deficit/hyperactivity disorder (ADHD) or autism spectrum disorder (ASD) in neuropediatric clinics. Methods: Participants were children and adolescents (N = 337) referred to neuropediatric outpatient clinics for neurodevelopmental assessment. Functional impairment was evaluated using three instruments: the SDQ impact, the Vineland Adaptive Behavior Scale (VABSII), and the Children's Global Assessment Scale (CGAS). Mental health symptoms and intellectual function were also assessed. We investigated convergent and concurrent validity of the SDQ impact. Results: The convergent validity of the SDQ impact was shown by its significant correlations with the VABS-II composite score and the CGAS total score. The concurrent validity of the SDQ impact was demonstrated by its significant relationship with ADHD and ASD diagnoses in logistic regression analyses. Using established cutoffs, the sensitivity of the SDQ impact to reveal functional impairment in children with ADHD and ASD diagnoses was demonstrated in this neuropediatric sample, but at the cost of low specificity. Conclusion: The SDQ impact is an easy-to-use tool, and the overall study results indicate that it is partially valid, suggesting it may be used for the screening of general functional impairment in the neuropediatric population.