Ptosis is a common ocular complaint in the elderly population. While most cases are often due to involutional changes in the eyelid, neurogenic, mechanical or myogenic diseases should remain as important differential diagnoses. We report a case of a 75-year-old Chinese man who presented with chronic progressive ptosis and was initially managed as presumed seronegative ocular myasthenia gravis (MG). Despite positive bedside findings suggestive of MG, serological and electrophysiological investigations were inconclusive, and there was no clinical response to pyridostigmine. The diagnosis was achieved only after an opportunistic frontalis muscle biopsy obtained during planned brow lift surgery demonstrated histopathological features supportive of a mitochondrial aetiology. This case highlights the diagnostic challenge of distinguishing mitochondrial myopathies (MM) from ocular MG, emphasises clinical features that should prompt reconsideration of the diagnosis, and illustrates how opportunistic biopsy during oculoplastic surgery may facilitate diagnosis in selected patients with persistent diagnostic uncertainty.