Background Risk stratification for adults with congenital heart disease is usually based on the anatomic complexity of the patients’ defect. The 2018 American Heart Association/American College of Cardiology guidelines for the management of adults with congenital heart disease proposed a new classification scheme, combining anatomic complexity and current physiological stage of the patient. We aimed to investigate the capacity of the Adult Congenital Heart Disease Anatomic and Physiological classification to predict 15‐year mortality. Methods and Results Data on 5 classification systems were collected for 629 patients at the outpatient clinic for a previous study. After 15 years, data on mortality were obtained through medical record review. For this assessment, we additionally collected information on physiological state to determine the Adult Congenital Heart Disease Anatomic and Physiological classification. Harrell's concordance statistics index, obtained through a univariate Cox proportional hazards regression, was 0.71 (95% CI , 0.63−0.78) for the Adult Congenital Heart Disease Anatomic and Physiological classification. Harrell's concordance statistics index of the congenital heart disease anatomic component only was 0.67 (95% CI , 0.60−0.74). The highest Harrell's concordance statistics index was obtained for the anatomic complexity in combination with the Congenital Heart Disease Functional Index (0.79; 95% CI , 0.73–0.84). Conclusions This first investigation of the Adult Congenital Heart Disease Anatomic and Physiological classification system provides empirical support for adding the physiological component to the anatomic complexity in the prediction of 15‐year cardiac mortality.