Patients with Duchenne muscular dystrophy have a higher incidence of neurodevelopmental disorders, particularly autism spectrum and attention-deficit/hyperactivity disorders, than the general population. However, the clinical outcome assessments used in the evaluation of patients with Duchenne and as endpoints in clinical trials require significant patient cooperation, which can be challenging in those with severe behavioral issues and leads to their exclusion. With ongoing expansion of the therapeutic arsenal, we aimed to explore differences in motor function test completion and measurements between Duchenne patients with and without autism spectrum disorder or attention-deficit/hyperactivity disorder in a large tertiary care hospital's pediatric neuromuscular clinic. We identified significantly lower rates of motor function testing and motor function test scores among Duchenne patients with autism or attention-deficit/hyperactivity disorder. These findings underscore the need for adequate opportunity to complete motor function testing in those patients. Alternatively, the Duchenne community could consider validating more patient-reported outcomes and wearable device outcome measures in trials that families of children with neurodevelopmental disorders could more easily use. These interventions would improve equitable access to new therapies for patients with severe behavioral issues and allow researchers to track broader clinical outcomes among all patients as the Duchenne treatment landscape expands.