Introduction The relationship between the duration of diabetes mellitus and coronary artery disease (CAD) is well established. Moreover, the Gensini score system is a reliable assessment tool for the severity of coronary artery disease (CAD). After an extensive literature search, we found that there is a knowledge gap about the relationship between the Gensini score and the duration of diabetes in our population. Therefore, the aim of this study was to find the relationship between the Gensini score and the duration of diabetes in patients undergoing a coronary angiography. Methods A cross-sectional study was conducted among 321 consecutively selected diabetic patients. Clinically diagnosed cases of diabetes on proper anti-diabetic treatment were included in this study. Patients with known severe CAD or history of coronary artery bypass grafting (CABG) surgery or primary coronary intervention (PCI) were excluded from the study. Coronary angiography was performed on all the patients and their Gensini score was calculated using the modified scoring schema. Duration of diabetes and other baseline risk factors were recorded for all patients. The relationship between the Gensini score and the duration of diabetes was assessed by calculating Pearson’s correlation coefficient. Results A total of 321 diabetic patients were included in this study, out of which 67.9% (218) were men and mean ± standard deviation age was 56.13 ± 7.67 years ranging between 40 to 70 years with a majority of the patients, 63.9% (205), being under 60 years of age. Along with diabetes, the most commonly observed risk factor was hypertension, which was observed in 77.6% (249) of the patients. Smoking and obesity were also observed in 18.7% (60) and 17.4% (56) of the patients, respectively. A significant positive correlation, 0.55 (p<0.001), was observed between the duration of diabetes and the Gensini scores. The correlation was found to be stronger in older patients (more than 60 years of age) with a correlation coefficient of 0.52 vs. 0.38, and male patients with correlation coefficients of 0.66 vs. 0.34. Conclusion A significant positive correlation between the Gensini score and the duration of diabetes mellitus was observed. This correlation is relatively stronger among male and older patients (more than 60 years of age).