Childhood-onset myasthenia gravis (MG), which is more common in children under 5 years of age than in adolescents, is generally characterized by ocular symptoms and an absence of anti-acetylcholine receptor (AChR) and anti-muscle-specific tyrosine kinase (MuSK) antibodies. We treated a 13-year-old boy presenting with a chief complaint of ptosis after COVID-19 infection. Magnetic resonance imaging of the head showed no brain abnormalities, and no anti-AChR or anti-MuSK antibodies were detected. Nor did repetitive nerve stimulation reveal any abnormal decrement. However, a Tensilon test proved positive for MG, leading to a diagnosis of ocular MG. Treatment with a cholinesterase inhibitor brought about no improvement in the patient's symptoms. Prednisolone was administered to treat the diplopia caused by his ocular motility disorder, again with no symptomatic relief. By contrast, steroid pulse therapy was effective in resolving the patient's drooping eyelids and ocular motility issues. No recurrence of symptoms was observed after prednisolone was discontinued. Of note, the exacerbation of the patient's symptoms coincided with the detection of previously undetected AChR antibodies. Routine electromyography and testing for anti-AChR antibodies may prove useful in selecting treatment and predicting outcomes in patients with adolescent MG.