This study presents construct validity for a novel automated morphometric and kinematic handwriting assessment, including (1) convergent validity, establishing reliability of automated measures with traditional manual-derived Minnesota Handwriting Assessment (MHA), and (2) discriminant validity, establishing that the automated methods distinguish populations of children with previously reported impairments in handwriting. Results revealed standard (manual derived) MHA, and comparable automated handwriting scores were highly correlated, providing evidence for convergent validity. Participants in the discriminant validity study included 167 children (ages 7-12 years old; 65 typically developing [TD], 45 attention-deficit/hyperactivity disorder [ADHD], and 57 autism spectrum disorder [ASD]). Repeated measures ANOVAs were employed to test for diagnostic differences across conditions and groups; follow-up correlations using letter form were conducted to identify associations between handwriting and neuropsychological performance. Diagnostic group comparisons provided evidence of discriminant validity, revealing elevated letter-form scores (worse performance) in children with ASD and ADHD groups compared to TD children; children with ASD showed the greatest letter-form impairment. These results provide strong evidence of construct validity for this novel automated handwriting assessment.