Background: Whether serum magnesium has a role in the treatment of chronic obstructive pulmonary disease (COPD) patients is still up for debate. Magnesium levels in COPD patients and the frequency of exacerbations were examined in this study, which also attempted to identify a possible cut-off point for diagnosing an exacerbation. Patients and Methods: This prospective observational study included 150 subjects, 30 of whom appeared healthy, and 120 of whom had COPD patients who visited the Benha University Hospital Chest Department between May 2021 and February 2022 for follow-up. Demographics data, clinical manifestations, and the number of admissions in previous year of all participants were collected together with measuring of serum magnesium levels. Results: There were 60 stable COPD patients (55 males and 5 females) and 60 acute exacerbated COPD (AECOPD) patients (58 males and 2 females). Stable COPD patients had a mean serum magnesium level of 2.04 (SD 0.05) mEq/L, while patients with AECOPD had levels that were 1.61 (SD 0.07) mEq/L, with statistically significant difference. Receiver operating characteristic curve showed a cut off value of 1.91 mg/dL of serum magnesium to diagnose exacerbation. Conclusion: Hypomagnesemia is common in COPD patients who are experiencing flare-ups, which increases their risk of relapse, their frequency of flare-ups, and their need for hospitalization. Magnesium, therefore, is a risk factor for COPD exacerbation that is both independent and adjustable.