This study compares early and late outcomes in patients undergoing coronary artery bypass grafting with and without preoperative atrial fibrillation in a contemporary, nationally representative Medicare cohort. In the Medicare-Linked Society of Thoracic Surgeons database, 361,138 patients underwent isolated coronary artery bypass from 2006 to 2013, of whom 37,220 (10.3%) had preoperative atrial fibrillation; 13,161 (35.4%) were treated with surgical ablation and were excluded. Generalized estimating equations were used to compare 30-day mortality and morbidity. Long-term survival was summarized using Kaplan-Meier curves and Cox regression models. Stroke and systemic embolism incidence was modeled using the Fine-Gray model and the CHA2DS2-VASc score was used to analyze stroke risk. Median follow-up was 4 years. Preoperative atrial fibrillation was associated with a higher adjusted in-hospital mortality (odds ratio OR, 1.5; P < .0001) and combined major morbidity including stroke, renal failure, prolonged ventilation, reoperation, and deep sternal wound infection (OR, 1.32; P < .0001). Patients with preoperative atrial fibrillation experienced a higher adjusted long-term risk of all-cause mortality and cumulative risk of stroke and systemic embolism compared to those without atrial fibrillation. At 5 years, the survival probability in the preoperative atrial fibrillation versus no atrial fibrillation groups stratified by CHA2DS2-VASc scores was 74.8% versus 86.3% (score 1-3), 56.5% versus 73.2% (score 4-6), and 41.2% versus 57.2% (score 7-9; all P < .001). Preoperative atrial fibrillation is independently associated with worse early and late postoperative outcomes. CHA2DS2-VASc stratifies risk, even in those without preoperative atrial fibrillation.