Charcot-Marie-Tooth disease is a phenotypically and genetically heterogeneous group of disorders affecting both motor and sensory neurons. Exome sequencing has driven discovery of genes responsible for Charcot-Marie-Tooth disease with more than 70 genes now associated with this neuromuscular disease. The MARS gene was recently reported as the cause of Charcot-Marie-Tooth 2U, a slowly progressive axonal sensorimotor polyneuropathy with adult-onset reported in six patients. We report here a patient with a progressive, early childhood-onset, motor-predominant form of Charcot-Marie-Tooth disease. Exome sequencing identified a novel MARS variant (c.1189G>A; p.Ala397Thr) that was not present in her unaffected mother; her unaffected father was unavailable. Further studies using structural modeling and a yeast humanization assay support pathogenicity of the variant. Our study expands the phenotype of Charcot-Marie-Tooth 2U, while highlighting the utility of functional assays to evaluate variant pathogenicity.