Background: Magnesium (Mg) is an essential mineral that plays a vital role in various physiological processes, including enzyme regulation, neuromuscular function, and cardiovascular health. Dysmagnesemia has been associated with arrhythmias, neuromuscular dysfunction, and poor outcomes in intensive care unit (ICU) settings, representing diagnostic and therapeutic challenges. However, the relationship between dysmagnesemia and health outcomes in the ICU remains inadequately defined. Aim/Objective: This study aimed to assess the prevalence of dysmagnesemia and evaluate the correlation between total (tMg) and ionized magnesium (iMg) levels in a cohort of ICU and high dependency unit (HDU) patients. It also sought to evaluate patient characteristics and relevant health outcomes by comparing both concentrations of iMg and tMg. Methods: This prospective study was conducted among adult patients admitted to the ICU and the high dependency unit (HDU). Results: Among the 134 included patients, the median age was 63.5 years (IQR: 52.0–77.0). The majority, 91.0%, required mechanical ventilation. Additionally, 50.0% were diagnosed with diabetes, 28.4% had chronic kidney disease, and proton pump inhibitors (PPIs) were administered to 67.2% of the patients. The prevalence of hypomagnesemia, as measured by iMg, was 6.7%, while hypermagnesemia was at 39.6%. When measured by tMg, hypomagnesemia and hypermagnesemia were observed at rates of 14.9% and 22.4%, respectively. The iMg measurements showed an association between the incidence of atrial fibrillation and hypomagnesemia (p = 0.015), whereas tMg measurements linked hypomagnesemia with longer hospital stays. Notably, only a few patients identified with iMg-measured hypomagnesemia received magnesium replacement during their ICU stay. Conclusions: Dysmagnesemia is prevalent among critically ill patients, with discordance between iMg and tMg measurements. iMg appears more sensitive in detecting arrhythmia risk, while tMg correlates with length of stay. These findings support the need for larger studies and suggest considering iMg in magnesium monitoring and replacement strategies.