Fulminant presentations of acute myocarditis can predict cardiac functional and patient outcomes. This longitudinal study compared pediatric cohorts with fulminant Multisystem Inflammatory Syndrome-related myocarditis (MISCM) vs non-COVID-19 viral myocarditis (VM) to test the hypothesis that the adverse left ventricular (LV) remodeling rather than the phenotype of presentation predicts clinical outcomes. This is a retrospective analysis of 54 children with MISCM (age 6 ± 4 years, weight 32.5 ± 24.6 kg, male 44%) and 26 children with VM (age 3.8 ± 4.8 years, weight 17.7 ± 17.7 kg, male 56%) on hospitalization and one-year follow-up. VM patients exhibited acute LV remodeling, but MISC patients did not (LV end-diastolic dimension z score 2.05 ± 2.50 vs 0.04 ± 1.10, p = 0.00). Compared to the MISCM, VM patients had severe LV systolic and diastolic dysfunction (ejection fraction 54.6% vs 39.9%, four-chamber longitudinal strain - 15.6% vs - 8.7%, and left atrial strain 25.5% vs 13.9% p = 0.000), increased need for mechanical circulatory support (39% vs 7%), 2 mortalities, one cardiac transplant, and stage C heart failure in the 17 survivors at discharge. Ejection fraction normalized but abnormal segmental four-chamber longitudinal strain persisted in both cohorts with most VM patients remaining on anti-failure treatment at one-year follow-up. Inflammation-mediated acute LV remodeling rather than the phenotype of presentation may determine LV function and patient outcomes. Non-invasive imaging can play a useful role in the assessment of the mechanism of LV remodeling and defining the trajectory of LV function and cardiac outcomes.