Introduction Congenital myotonic dystrophy (CDM) is a rare neuromuscular disorder characterized by severe hypotonia and muscle weakness at birth that tends to improve with age. Understanding lean body and bone mass in this population has important research and clinical implications. The main objective of this study was to determine whether older children with CDM had muscle mass similar to healthy controls. Methods Thirty‐five patients with CDM (3‐13 years old) were enrolled. We analyzed lean body mass (LBM) and bone mineral content using the mechanostat framework, which allows calculation of z‐scores for sex, age, and height. Results All patients had low LBM z‐scores (muscle mass); however, higher LBM z‐score was positively correlated with age (r = 0.45, P = 0.006), showing that it is closer to normal in older patients. Discussion Unlike other neuromuscular diseases, older children with CDM have a muscle mass closer to age‐matched controls, consistent with the clinical profile of increasing strength in childhood. Muscle Nerve 60: 176–179, 2019