Nucleotide repeat sequences are prevalent in the genome and expansion of these sequences is associated with more than 40 neuromuscular disorders. To understand the pathogenic mechanisms underlying RNA-repeat toxicity, we performed a genetic screen in a Caenorhabditis elegans model expressing an expanded CUG repeat specifically in the muscle. Here, we show that expression of this RNA repeat impairs motility by mitochondrial dysfunction, disrupting mitochondrial morphology and respiration. The phenotype is dependent on the RNA-binding factor MBL-1 and requires factors from the ribosome associated protein quality control complex. Furthermore, Coenzyme Q supplementation rescued the motility impairment and all of the mitochondrial phenotypes. Together, our data reveal the importance of mitochondrial dysfunction in the molecular pathogenesis of RNA repeat expansion disorders.