Objective: The Modified Barthel Index (MBI) measures individuals’ level of independence in performing activities of daily living. The purpose of this study was to examine the internal consistency and construct validity of the MBI for children with rare disorders. Methods: The study participants are children with rare disorders who have muscle weakness including Barth syndrome and congenital muscular dystrophy (N = 113). The MBI was completed by participants in either an online format utilizing the UF Qualtrics system or a pencil-and-paper format. Statistical analysis was conducted to examine the psychometric properties of the MBI. Results: The MBI showed excellent internal consistency for children with Barth syndrome (Cronbach’s α = .91) and congenital muscular dystrophy (Cronbach’s α = .93). Construct validity was supported by a significant difference in MBI scores among participants grouped by diagnoses. MBI score was significantly different between the unaffected group and congenital muscular dystrophy (p < .0001) as well as between the unaffected group and Barth syndrome (p < .0001). Conclusion: The psychometric properties of the MBI present good reliability and construct validity suggesting suitability for use as an outcome measure for children with rare disorders.