Background Among patients with an implantable cardioverter‐defibrillator, a high prevalence of atrial fibrillation (AF) is present. Identification of AF predictors in this patient group is of clinical importance to initiate appropriate preventive therapeutic measures to reduce the risk of AF‐related complications. This study assesses whether cardiac magnetic resonance imaging–derived atrial characteristics are associated with AF development in patients with a dual‐chamber implantable cardioverter‐defibrillator or cardiac resynchronization therapy defibrillator, as detected by the cardiac implantable electronic device. Methods and Results This single‐center retrospective study included 233 patients without documented AF history at the moment of device implantation (dual‐chamber implantable cardioverter‐defibrillator [63.5%] or cardiac resynchronization therapy defibrillator [36.5%]). All patients underwent cardiac magnetic resonance imaging before device implantation. Cardiac magnetic resonance–derived features of left atrial (LA) remodeling were evaluated in all patients. Detection of AF episodes was based on cardiac implantable electronic device interrogation. During a median follow‐up of 6.1 years, a newly diagnosed AF episode was detected in 88 of the 233 (37.8%) patients with an ICD. In these patients, increased LA volumes and impaired LA function (LA emptying fraction and LA strain) were found as compared with patients without AF during follow‐up. However, a significant association was only found in patients with dilated cardiomyopathy and not in patients with ischemic cardiomyopathy. Conclusions LA remodeling characteristics were associated with development of AF in patients with dilated cardiomyopathy but not patients with ischemic cardiomyopathy, suggesting different mechanisms of AF development in ischemic cardiomyopathy and dilated cardiomyopathy. Assessment of LA remodeling before device implantation might identify high‐risk patients for AF.