We experienced a case in which mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) was identified as complications following the onset of Leigh syndrome along with a 10191 T>C mutation of the mitochondrial gene. The case pertains to a 26-year-old woman. The disease appeared when she was 11 years old due to divergent strabismus, at which point a diagnosis of juvenile Leigh syndrome was made. Many infraction images not conforming to the vessel region were observed upon a brain MRI which was performed at 26 years of age, thus leading to her being diagnosed with MELAS as a complication. Upoon bibliographical consideration, it was speculated that the clinical features of MELAS/Leigh overlap syndrome clearly differ from Leigh syndrome in terms of age of onset, symptoms, and prognosis. Pleiotropic genetic factors including heteroplasmy were presumed to be involved in the diverse phenotype of overlap syndrome.