Abstract Aims The aim of this study is to investigate the prognostic implications of the presence of heart failure (HF) across the range of left ventricular ejection fraction (LVEF) in patients with comorbid atrial fibrillation (AF). Methods and results We conducted a retrospective cohort study of 1063 patients (median age 76 years), discharged from the cardiology ward with a primary or secondary diagnosis of AF between 2015 and 2018. We used Cox proportional‐hazards and spline models to examine the association of the presence of HF, across the range of LVEF, with the primary outcome of all‐cause mortality. HF was documented in 52.9% of patients at baseline. During a median follow‐up of 31 months (interquartile range 10 to 52 months), 37.3% of patients died. The presence of HF was associated with a significantly higher risk of mortality [adjusted hazard ratio (aHR) 2.17; 95% confidence interval (CI), 1.70 to 2.77; P