RASopathies are a group of neurodevelopmental disorders caused by germline mutations that result in hyperactivation of the RAS/MAPK pathway. These syndromes, which include Costello Syndrome (CS) caused by activating mutations in HRAS, share clinical manifestations such as neurocognitive impairment, cardiac defects, musculoskeletal abnormalities, and increased cancer risk. Emerging evidence suggests that metabolic rewiring contributes to disease pathophysiology, yet the underlying mechanisms remain poorly understood. Here, we identify a paradoxical metabolic phenotype in a CS mouse model characterized by reduced white adipose tissue (WAT) and increased lean mass despite impaired physical performance. RNA-seq and imaging analyses support a model in which lipids are inefficiently stored in adipose tissue and instead redistributed to peripheral tissues, including skeletal muscle, where lipid accumulation is observed. At the cellular level, skeletal muscle exhibits profound mitochondrial abnormalities, including disrupted cristae and altered morphology. HRAS G12S-expressing myoblasts recapitulate these defects, showing impaired oxidative metabolism, altered membrane potential, and intracellular lipid accumulation. Together, these findings indicate that mitochondrial dysfunction drives lipid redistribution and metabolic inefficiency in CS, revealing a potential mechanism of impaired energy storage and identifying new opportunities for metabolic intervention.