Essential thrombocythemia (ET) is a myeloproliferative disorder characterized by hemorrhagic and thrombotic complications. We describe a rare case of ST-segment elevation myocardial infarction (STEMI) in a patient with previously undiagnosed ET, confirmed by gene mutation. A 68-year-old man presented with severe acute chest pain and was diagnosed with STEMI. Primary coronary angiography showed severe stenosis with thrombus in the proximal left anterior descending coronary artery. Percutaneous aspiration thrombectomy was performed with no residual stenosis. The patient was discharged on antiplatelet agents, aspirin, and clopidogrel. Further investigations for intracoronary thrombus with no underlying atherosclerotic disease revealed positive Janus kinase 2 (JAK2) V617F gene mutation, and this was consistent with a diagnosis of ET with elevated platelet count. This case describes a rare initial presentation of previously undiagnosed ET with acute STEMI and highlights the potential importance of secondary workup for non-atherosclerotic causes of STEMI with isolated intracoronary thrombus otherwise normal coronary vasculature with no focal atherosclerosis.