FHL1 mutations cause several clinically heterogeneous myopathies, including reducing body myopathy (RBM), scapuloperoneal myopathy (SPM) and X-linked myopathy with postural muscle atrophy (XMPMA). The molecular mechanisms underlying the pathogenesis of FHL1 myopathies are unknown. Protein aggregates, designated ‘reducing bodies’, that contain mutant FHL1 are detected in RBM muscle but not in several other FHL1 myopathies. Here, RBM, SPM and XMPMA FHL1 mutants were expressed in C2C12 cells and showed equivalent protein expression to wild-type FHL1. These mutants formed aggregates that were positive for the reducing body stain Menadione-NBT, analogous to RBM muscle aggregates. However, hypertrophic cardiomyopathy (HCM) and Emery-Dreifuss muscular dystrophy (EDMD) FHL1 mutants generally exhibited reduced expression. Wild-type FHL1 promotes myoblast differentiation; however, RBM, SPM and XMPMA mutations impaired differentiation, consistent with a loss of normal FHL1 function. Furthermore, SPM and XMPMA FHL1 mutants retarded myotube formation relative to vector control, consistent with a dominant-negative or toxic function. Mutant FHL1 myotube formation was partially rescued by expression of a constitutively active FHL1-binding partner, NFATc1. This is the first study to show that FHL1 mutations identified in several clinically distinct myopathies lead to similar protein aggregation and impair myotube formation, suggesting a common pathogenic mechanism despite heterogeneous clinical features.