Abstract Background Takayasu arteritis is a large vessel vasculitis that affects the aorta and its primary branches. Myocarditis is a rare life-threatening complication and potential diagnostic pitfall in patients with Takayasu arteritis. Case Summary A previously healthy 18-year-old woman presenting with fever, back pain, and dyspnea was admitted to another hospital for acute hypertension (blood pressure, 230/106 mmHg) and congestive heart failure. Intravenous methylprednisolone pulse with antihypertensive and diuretic medications slightly improved her congestion. However, she developed acute kidney injury and was transferred to our hospital. Transthoracic echocardiography indicated a left ventricular ejection fraction of 45% and diffuse left ventricular hypokinesis. Doppler ultrasound test and magnetic resonance angiography revealed severe bilateral renal-artery stenosis. Her diagnosis was Takayasu arteritis and she received high-dose glucocorticoids. She required temporary hemodialysis, but two months after admission, her serum creatinine improved to 1.1 mg/dL without surgical/cardiovascular interventions. Although the pre-discharge test with 1.5- Tesla cardiac magnetic resonance initially failed to diagnose myocarditis, 3-Tesla cardiac magnetic resonance imaging revealed increased native T1 values on T1 mapping (1283–1393 ms), moderate pericardial effusion, and systolic left ventricular wall motion abnormality, indicating active myocarditis. During 6-month subcutaneous tocilizumab treatment (162 mg/week), a left ventricular ejection fraction improved to 55–60% without a relapse. Discussion This case report highlights the benefits of early multimodal imaging tests including cardiac magnetic resonance imaging for myocarditis and renal artery involvement in Takayasu arteritis. Tocilizumab might be an efficient therapeutic option for severe acute manifestations including myocarditis in young women of reproductive age.