Background: Type 1 diabetes mellitus (T1DM) predisposes individuals to cardiovascular complications, including diastolic dysfunction, even before overt heart disease manifests. Hemoglobin A1c (HbA1c) serves as a marker for long-term glycemic control and may influence cardiac function in T1DM. Methods: This cross-sectional observational study included 233 T1DM cases and 238 healthy controls to investigate the association between HbA1c levels and left ventricular diastolic function. Echocardiography assessed parameters including E/A ratio, E/e' ratio, left atrial volume index (LAVI), IVRT, DT, LV mass index, and LVEF. Statistical analyses included t-tests, correlation coefficients, and multivariate regression. Results: T1DM cases exhibited significantly impaired diastolic function compared to controls, as indicated by lower E/A ratio (0.85 vs. 1.20) and higher E/e' ratio (10.5 vs. 8.1), LAVI (28.4 vs. 24.3 mL/m²), IVRT (92.5 vs. 84.6 ms), DT (210.7 vs. 190.3 ms), LV mass index (95.2 vs. 88.1 g/m²), and slightly lower LVEF (62.5% vs. 64.1%). Elevated HbA1c levels correlated significantly with worsened diastolic parameters: negative correlation with E/A ratio (r = -0.42) and LVEF (r = -0.22), and positive correlations with E/e' ratio (r = 0.39), LAVI (r = 0.35), IVRT (r = 0.30), DT (r = 0.28), and LV mass index (r = 0.25) (all p < 0.001). Conclusion: Higher HbA1c levels are associated with impaired left ventricular diastolic function in T1DM patients without overt heart disease, highlighting the importance of glycemic control in mitigating early cardiac dysfunction. Regular monitoring and optimization of glycemic control may prevent the progression to symptomatic heart disease in T1DM.