Background: Recent randomised clinical trials have suggested prognostic benefits of catheter ablation in highly selected patients with atrial fibrillation (AF) and heart failure (HF). Objectives: This study sought to identify the treatment effect associated with catheter ablation in a broad population of patients with AF and HF. Methods: Through nationwide administrative registers in Denmark, we estimated the 2-year average treatment effect (ATE) of catheter ablation for AF on a composite endpoint of HF readmission, stroke and all-cause mortality at 1-year and 5-year landmark analyses. The primary cohort was patients with AF before HF, and the second cohort of patients with HF before AF. Results: A total of 13 756 patients were included with 9904 patients in the primary cohort, and 3852 in the secondary. An ATE (95% CI) reduction of the composite endpoint of 7.0% (4.5% to 9.5%) was observed in the primary cohort and 11.8% (6.0% to 17.6%) in the secondary in the 1-year landmark analysis with a reduction in all-cause mortality of 5.8% (3.7%-7.8%) and 6.3% (0.9%-11.7%), respectively. At the 5-year landmark, catheter ablation was associated with reductions in the composite endpoint and all-cause mortality in the primary (4.7% (2.3% to 7.2%), and 3.6% (1.0% to 6.3%), respectively), but not in the secondary cohort. Conclusions: Ablation was associated with decreased risk of HF readmission, stroke and all-cause mortality in patients with AF and HF. The effect is most substantial in patients with AF before HF and with catheter ablation after 1 year from the diagnosis of both conditions.