Background <p>Epidemiological studies have discovered an inverse correlation between serum Mg²⁺ concentration and the risk of developing heart failure (HF) as well as atrial fibrillation (AF) in patients with type 2 diabetes mellitus (T2DM). The prognostic relevance of serum Mg²⁺ concentration in patients with HF and T2DM remains unclear.</p> Objectives <p>To assess the association of serum Mg²⁺ concentration with cardiovascular event rates in a retrospective cohort of patients with HF and T2DM.</p> Methods <p>This study included patients diagnosed with HF and T2DM in First Teaching Hospital of Tianjin University of Traditional Chinese Medicine, Tianjin, from July 2018 to December 2021. The primary endpoint was cardiovascular death or rehospitalization for HF.</p> Results <p>This study included 216 patients with HF and T2DM. Patients were divided into three groups (T1 [serum Mg²⁺ level ≤ 0.8 mmol/L], T2 [0.8 &lt; serum Mg²⁺ level ≤ 0.89 mmol/L], T3 [serum Mg²⁺ level ≥ 0.9 mmol/L]) based on serum Mg²⁺ level. The Kaplan-Meier analysis revealed that the incidence rate of primary endpoint was significantly increased among three groups (<i>P</i> &lt; 0.001). In multivariate Cox regression model, elevated serum Mg²⁺ level was considered as a protective factor in the coexistence of T2DM and HF with poor prognosis.</p> Conclusions <p>Our results indicated that increased serum Mg²⁺ concentration may improve long-term prognosis of patients with HF and T2DM.</p>