Hemoglobin A1c (HbA1c) levels are used as a measure of glycemic control, with greater levels indicating poorer control and a greater risk of death. However, recent studies have found a U-shaped association between the HbA1c levels and mortality in patients with heart failure, with the lowest risk of death associated with elevated HbA1c levels, usually >7%. Cardiac surgery is frequently used to mitigate the signs and symptoms of heart failure. The purpose of the present study was to determine the association between HbA1c levels and late mortality in cardiac surgery patients with and without heart failure. Patients with and without New York Heart Association class III or IV heart failure were divided into quartiles according to the preoperative HbA1c level. Mortality was determined for each group and compared using chi-square tests and Cox modeling. Of the 311 patients with heart failure, 65 (21%) were dead at follow-up compared to 57 of 669 patients (9%) without heart failure (p <0.001). After adjusting for confounders, the patients without heart failure and with HbA1c ≤5.7% had the lowest risk of death. In patients with preoperative heart failure, we found a U-shaped association between HbA1c levels and late mortality, with those patients with HbA1c levels of 5.8% to 6.2% having the lowest risk of death. HbA1c levels ≤5.7% and ≥7.2% were associated with statistically significant greater risks of death. In conclusion, we found in patients with heart failure that the lowest risk of death was associated with HbA1c levels of 5.8% to 6.2%.