Retrospective cohort study. To evaluate the association of Hemoglobin A1C (HbA1c) with surgical site infection (SSI) and patient-reported outcomes (PROs), and to identify optimal HbA1c thresholds to minimize the risk of SSI and maximize PROs. Diabetes mellitus has been associated with worsened outcomes following spine surgery. HbA1c, a surrogate of glycemic control, is an important assessment tool in diabetic patients. A single-center, retrospective cohort study using prospectively collected data was undertaken. Diabetic patients undergoing elective lumbar decompression surgery between October 2010 and May 2021 were included. HbA1c, demographics, comorbidities, and perioperative data were collected. Primary outcomes included: 1) SSI, and 2) PROs, including the Numeric Rating Scale (NRS)-back/leg pain and Oswestry Disability Index (ODI). Secondary outcomes included: complications, readmissions, and reoperations within 90-days postoperatively. The minimum clinically important difference (MCID) was set at a 30% improvement from baseline PROs. Of 1819 patients who underwent lumbar decompression surgery, 368 patients had diabetes mellitus, and 177 had a documented preoperative HbA1c value. Of patients with available HbA1c values, the mean age was 62.5±12.3, the mean HbA1c value was 7.2±1.5%, and SSI occurred in 3 (1.7%) patients only, which prevented further analysis of SSI and HbA1c. A significant association was seen with a higher HbA1c and failure to achieve NRS-Back pain MCID30 Odds ratio(OR)=0.53, 95% confidence interval(CI) 0.42-0.78; P =0.001 and ODI MCID30 (OR=0.58, 95%CI 0.44-0.77; P =0.001), but not NRS-Leg pain MCID30 (OR=1.29, 95%CI 0.86-1.93; P =0.208). ROC-curve analysis and Youden's index revealed an HbA1c threshold of 7.8 for NRS-Back pain MCID30 (AUC=0.65, P <0.001) and 7.5 for ODI MCID30 (AUC=0.65, P =0.001). In diabetic patients undergoing elective lumbar decompression surgery, HbA1c levels above 7.8 and 7.5 were associated with less improvement of NRS-Back and ODI scores at 12-months postoperatively, respectively. To optimize PROs, We recommend a preoperative HbA1c of 7.5 or below for diabetic patients undergoing elective lumbar decompression surgery.