This case report describes a rare case of progressive muscle weakness in a patient treated for eosinophilic fasciitis (EF) for many years before being diagnosed with a second autoimmune disease: dermatomyositis. Our case is a report of a 65-year-old male diagnosed with eosinophilic fasciitis 7 years before being evaluated in our service at Mayo Clinic in Jacksonville, Florida, due to progressive muscle weakness despite the chronic treatment with methotrexate. Contrast-enhanced magnetic resonance imaging of the lower extremity showed enhancement throughout the thigh musculature, which led us to pursue biopsies of the fascia and muscle in order to confirm the diagnosis of EF associated with myopathy. This case illustrates the need to consider the possibility of myopathy in patients diagnosed with EF whenever muscle weakness is more prominent than expected.