Summary Background Type 2 diabetes mellitus (T2DM) is a common disease; the total number of diabetes patients is expected to reach 366 million by 2030. Magnesium has received considerable attention for its potential role in improving insulin sensitivity and preventing diabetes and its cardiovascular complications. Hypomagnesaemia is linked to poor control of type 2 diabetes mellitus, and depletion of serum magnesium occurs exponentially with duration of disease. The aim of this study was to measure serum magnesium level and the correlation of magnesium level with HbA1c in type 2 diabetes mellitus. Methods In this cross‐sectional study, we included 100 diabetic patients; blood is taken for measurement of both magnesium and HbA1c levels. Patients with risk factors of magnesium deficiency were not included. Results Of the total 100 patients recruited in our study, majority had diabetes for 8.0 years duration. The mean age of the patients was 53.97 ± 10.65 years, most of them were on oral hypoglycaemic agents (84.9%) followed by combination of oral hypoglycaemic agents and insulin (10.5%), and a small percentage were on insulin alone (4.7%). The mean of serum magnesium and HbA1c levels was 1.88 ± 0.25 mg/dL and 8.38% ± 1.53%, respectively. The majority of the patients had a normal level of magnesium (95.0%); however, most of them had uncontrolled blood glucose (82.0%). The study showed that the serum magnesium level and HbA1c are not significantly correlated ( P = 0.462). Conclusion Normal magnesium levels were observed in majority of patients, there is no significant correlation between serum magnesium and HbA1c levels in patients with T2DM, but larger‐scale clinical trials are needed in future.