Background/Objectives: Diabetes mellitus is associated with cognitive impairment, but evidence from southern Morocco remains limited. This study examined the associations of diabetes status, HbA1c, and diabetes duration with Mini-Mental State Examination (MMSE) performance among adults aged 45 years or older in Dakhla. Methods: This hospital-based comparative cross-sectional study included 100 participants (50 with diabetes and 50 without diabetes). Data were collected through a structured bilingual questionnaire and medical records. MMSE scores below 27 defined MMSE-based cognitive impairment. Crude and age-, sex-, and education-adjusted linear regression models with HC3 robust standard errors were complemented by correlation, subgroup, chi-square, and odds-ratio analyses. Results: Mean MMSE scores were lower in participants with diabetes than in those without diabetes (24.4 ± 5.39 vs. 28.8 ± 2.08). After adjustment, diabetes remained associated with a 4.13-point lower score (β = −4.13; 95% CI: −5.57 to −2.69; p < 0.001). MMSE-defined impairment occurred in 54% and 14% of the diabetic and non-diabetic groups, respectively (OR = 7.21; 95% CI: 2.72–19.05; p < 0.001). Within the diabetic group, MMSE scores correlated inversely with diabetes duration (r = −0.523; p < 0.001) and HbA1c (r = −0.393; p = 0.005), but neither remained independently associated after adjustment. Participants with type 1 diabetes had lower median MMSE scores and longer disease duration than those with type 2 diabetes, although this subgroup comparison was exploratory. Conclusions: Diabetes was strongly associated with lower MMSE performance and a higher prevalence of MMSE-defined impairment in this hospital-based sample. The cross-sectional design precludes causal interpretation, and the findings require confirmation in larger prospective studies.