Introduction The role of implantable cardioverter defibrillators (ICDs) in nonischemic cardiomyopathy is unclear and better risk‐stratification is required. We sought to determine if T1 mapping predicts appropriate defibrillator therapy in patients with nonischemic cardiomyopathy. We studied a mixed cohort of ischemic and nonischemic patients to determine whether different cardiac magnetic resonance (CMR) applications (T1 mapping, late gadolinium enhancement, and Grayzone) were selectively predictive of therapies for the different arrhythmic substrates. Methods and results We undertook a prospective longitudinal study of consecutive patients receiving defibrillators in a tertiary cardiac center. Participants underwent CMR myocardial tissue characterization using T1 mapping and conventional CMR scar assessment before device implantation. QRS duration and fragmentation on the surface electrocardiogram were also assessed. The primary endpoint was appropriate defibrillator therapy. One‐hundred thirty patients were followed up for a median of 31 months (IQR ± 9 months). In nonischemic patients, T1_native was the sole predictor of the primary endpoint (hazard ratio HR 1.12 per 10 millisecond increment in value 95% confidence interval CI 1.04–1.21; P ≤ 0.01). In ischemic patients, Grayzone_2SD‐3SD was the strongest predictor of appropriate therapy (HR 1.34 per 1% left ventricular increment in value 95% CI 1.03–1.76; P = 0.03). QRS fragmentation correlated well with myocardial scar core (receiver operating characteristic area under the curve ROC AUC 0.64; P = 0.02) but poorly with T1_native (ROC AUC 0.4) and did not predict appropriate therapy. Conclusions In the medium–long term, T1_native mapping was the only independent predictor of therapy in nonischemic patients, whereas Grayzone was a better predictor in ischemic patients. These findings suggest a potential role for T1_native mapping in the selection of patients for ICDs in a nonischemic population.