Mitral valve chordae rupture in infancy is a rare, but life-threatening disease. The progression of acute cardiac failure has been reported, with emergency surgery being required in most cases. Mitral valve chordae rupture typically occurs at the age of 4–6 months. Echocardiography is needed to diagnose this disease, and accurate diagnosis is difficult for general pediatricians. We herein describe the case of an acutely ill 4-month-old infant, who was saved and discharged without neurological sequelae due to the early diagnosis of mitral valve chordae rupture, life support, and surgery. We confirm the importance of acute pre-surgery treatment and immediate surgery for the survival and good outcome of infants.