The purpose of this study is to investigate the corneal biomechanical properties, optic nerve head (ONH) topographic parameters, and retinal nerve fiber layer (RNFL) thickness changes in primary open-angle glaucoma (POAG) patients with and without diabetes, as well as to evaluate the effect of the metabolic control of diabetes on corneal biomechanical properties, ONH topography, and RNFL thickness. A total of 101 eyes of 101 POAG patients (60 with diabetes and 41 without diabetes) were recruited in this prospective study. Corneal hysteresis and corneal resistance factor (CRF) were both measured using the ocular response analyzer. Optic disk parameters were evaluated using the Heidelberg Retina Tomograph-III. RNFL thickness was measured by using Spectralis HRA + OCT. CRF, mean rim area, and rim volume were found to be significantly higher in the diabetic group when compared with non-diabetic group (p = 0.01 for CRF, p = 0.04 for rim area and p = 0.02 for rim volume). ANCOVA analysis showed statistically insignificant effects of age, gender, MD, and PSD values over rim area and rim volume (p > 0.05). CRF was not significantly correlated with HbA1c levels (p > 0.05). Cup area (CA), cup volume (CV), and cup shape measure (CSM) were weakly correlated with HbA1c levels (r = 0.35 and p = 0.006 for CA; r = 0.32 and p = 0.01 for CV; r = 0.32 and p = 0.01 for CSM). The difference in mean RNFL thickness values between the groups was found to be insignificant (p > 0.05). The results of this study raise doubts whether or not diabetes does in fact shield POAG patients from glaucomatous optic nerve damage from various perspectives.