Background: Dystrophin-deficient cardiomyopathy is becoming the dominant cause of death in patients with Duchenne muscular dystrophy (DMD), but its developmental process remains elusive. This study aimed to assess the development of left ventricular (LV) dysfunction that mimics DMD pathologies in golden retriever muscular dystrophy (GRMD) dogs. Methods: Transthoracic echocardiography was sequentially performed in GRMD dogs (n = 23) and age-matched healthy littermates (n = 7) from 2 to 24 months old. Conventional, tissue Doppler imaging, and speckle-tracking echocardiography parameters were analyzed. Results: At 2 months of age, GRMD dogs showed a pathologic decrease in the subendocardial-subepicardial gradient of radial systolic myocardial velocity along with altered LV twist and longitudinal strain, all being aggravated with age (analysis of variance, P < .001). Receiver operator characteristic curve analysis showed good ability to discriminate normal from GRMD dogs. LV ejection fraction was significantly decreased in GRMD dogs starting from 9 months and reached a pathologic level (<50%) at 24 months. Conclusions: The development of cardiomyopathy in GRMD dogs was characterized by subendocardial dysfunction, altered LV twist, and reduced longitudinal strain at a very young age to overall LV dysfunction in adults with transmural dysfunction, reduced LV ejection fraction and diastolic abnormalities, and even heart failure. This indicates the necessity to evaluate LV transmural myocardial velocity gradient, twist, and longitudinal strain in the early childhood of DMD patients.