Myasthenia gravis (MG) and idiopathic inflammatory myopathy (IIM) are autoimmune diseases that affect the musculoskeletal system. The association of the two diseases is rare. Their management is different, so it is important to recognize the concomitant presentation. In this cross-sectional study, we study the presence of CK elevation, myositis-specific and myositis-associated antibodies (MSA/MAA), and vitamin D levels in a cohort of 101 MG patients. Electromyography, limb magnetic resonance imaging (MRI), and, in some cases, muscle biopsy were performed when IIM was suspected. We reviewed the patients' medical records to access the results of these tests if they had been performed previously. CK elevation was detected in 10 patients (9.9%). We identified one case of anti-Jo-1 antibody-positive polymyositis and two cases of possible myositis. MSA/MAA antibodies were not found in the patients with high CK levels, except for the one with anti-Jo-1-positive IIM. One patient with elevated CK levels had an overlapping muscular dystrophy. MSA/MAA antibodies were detected in 19 patients (18.8%). A total of 37% had high-titer antibodies and concomitant systemic autoimmune diseases, while 63% had low-titer antibodies, most of whom had no systemic autoimmune disease. Low serum vitamin D levels were found in 67.3% of patients. Comparison of myasthenia gravis composite (MGC) scores between patients with low and normal vitamin D levels did not show a statistically significant difference. Our results may raise awareness among neuromuscular specialists caring for MG patients of the possibility of associated myositis or other neuromuscular diseases and the need to assess vitamin D levels. Although deficiency was frequent, its impact on MG severity remains unclear, necessitating further investigation into its immunological relevance.