Introduction: Delayed neurocognitive recovery (DNR; neurocognitive disorder up to 30 days postoperative) and postoperative neurocognitive disorders (POCD; neurocognitive disorder 1–12 months postoperative) occur frequently after surgery, with diabetes mellitus (DM) suggested to contribute to this. This was a single-center prospective cohort study. The main aim of this study was to investigate the role of DM and preoperative hemoglobin A1c (HbA1c) in the development of POCDs after noncardiac surgery. Methods: Older adult patients ≥65 years of age scheduled for elective surgery were recruited. The Modified Telephone Interview for Cognitive Status questionnaire (TICS-M), a test of global cognitive functioning, was administered to determine cognition. Preoperative, 30-day postoperative, and 6-month postoperative cognition were compared for patients with and without DM. Cognitive decline was subdivided into mild (1 to 2 standard deviations below controls) and major (≥2 standard deviations below controls) DNR or POCD. Preoperative HbA1c levels were correlated with TICS-M scores. Results: We analyzed 102 patients [median (IQR [range]) age 72.0 (5 [68–74])]), who were divided into patients with DM (80 patients [78%]) and patients without DM (22 patients [22%]). Baseline cognitive function was similar for both groups. Repeated measures ANOVA showed that mean DM patient TICS-M scores decreased 30 days postoperative (F(2, 200) = 4.0, p = 0.02), with subsequent recovery 6-month postoperative, compared to stable TICS-M scores in non-DM patients. There were significantly more DM patients with DNR than non-DM patients (n = 11 [50%] vs. n = 14 [17.5%]; p = 0.031). There were no between-group differences in mild or major POCD. Higher preoperative HbA1c levels were significantly correlated with decreased 30-day Δcognition scores (F(1, 54) = 9.4, p = 0.003) with an R2 of 0.149 (β −0.45, 95% confidence interval: −0.735 to −0.154). Conclusions: Older adult patients with DM undergoing surgery have an increased risk of DNR compared to older adult non-DM patients, but no increased risk of POCD. In DM patients, higher preoperative HbA1c levels were associated with an increased risk of DNR.