Lambert–Eaton myasthenic syndrome (LEMS) is a presynaptic neuromuscular junction disorder, and dermatomyositis (DM) is an idiopathic inflammatory myopathy. LEMS and DM are uncommon conditions that can present similarly and are often associated with autoantibodies. Concomitant LEMS and DM have only been reported a few times, and most of those cases were paraneoplastic. We present the first reported case of a patient with antivoltage gated calcium channel antibody positive LEMS who subsequently developed DM with antitranscription intermediary factor 1-gamma (anti–TIF1-γ) antibodies. Interestingly, both conditions occurred without evidence of malignancy. This diagnosis of LEMS and DM with characteristic clinical, electrodiagnostic, and histopathological evidence led to a beneficial modification of the patient’s therapeutic regimen. Due to the fact that overlapping concurrent neuromuscular conditions are rare, a high clinical suspicion is needed to identify, evaluate (including appropriate cancer screenings), and appropriately treat these patients.