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Background Cardiac MRI (CMR) derived ventricular global function index (GFI), a ratio of stroke volume to the sum of mean ventricular cavity and myocardial volumes, has demonstrated improved prediction of clinical outcomes in adults with atherosclerotic disease over ejection fraction. We sought to assess CMR derived GFI and a novel modification that accounts for unique loading conditions in patients with repaired tetralogy of Fallot (rTOF) and determine its correlation with exercise performance. Methods and Results Seventy‐five patients with rTOF who underwent CMR were identified. Clinical variables were recorded and biventricular GFI calculated. A right ventricular (RV) effective GFI (eGFI) was derived by incorporating effective stroke volume. Thirty‐five pediatric patients were matched with 29 age‐matched healthy controls. Twenty‐five patients completed cardiopulmonary exercise tests within 6 months of CMR. Stepwise regression models were used to determine univariate and multivariable predictors of indexed and percent predicted peak VO2. Median age at CMR was 20 years (interquartile range, 13–28). Pediatric rTOF patients had lower RV eGFI (P < 0.001), RV ejection fraction (P=0.002), but higher indexed RV end‐diastolic and end‐systolic volumes (P < 0.001, P < 0.001) compared with controls. Univariate analysis demonstrated a correlation between indexed peak VO2 with RV eGFI (R2=0.32, P=0.004), but with neither RVGFI, RV ejection fraction, indexed RV volumes nor RV mass. RV eGFI remained significantly associated with indexed peak VO2 during multivariable modeling. Conclusions Reduced RV eGFI was associated with reduced exercise capacity in rTOF patients, while RV GFI, RV ejection fraction, indexed RV volumes and mass were not. Our modification of the GFI, RV eGFI, may be a valuable non‐invasive marker of cardiac function in rTOF.