Serum magnesium is a frequently measured and treated electrolyte. However, few studies have examined magnesium level abnormalities and outcome in critically ill patients. Our objective was to determine the epidemiology and outcome of magnesium abnormalities among patients admitted to intensive care units (ICU). A retrospective cohort including adult patients admitted to three ICUs in southeast Queensland was assembled. Magnesium levels < 0.7, 0.7−1.0, and > 1.0 mmol/L were classified as low, normal, and high, respectively. Among 14,101 patients, the median age was 59.3 (interquartile range; IQR, 45.1–70.5), 7493 (56.4%) were male, and the median APACHE III score was 48 (IQR, 34–66). At admission, 3357 (23.8%) patients were classified as having hypomagnesemia, 1682 (11.9%) hypermagnesemia, 165 (1.2%) mixed, and 8897 (63.1%) as normal. Patients with magnesium abnormalities were more likely to be underweight and to have higher APACHE III scores. The overall 30-day case fatality was 8.2% (1155/14,101). Compared to those with normal levels, patients with hypermagnesemia at admission were at two-fold increased crude risk for death (relative risk; RR, 2.09; 95% confidence interval; CI, 1.83–2.39; p < 0.0001). After controlling for confounding variables in logistic regression analysis, neither admission hypo- nor hypermagnesemia was associated with death. However, development of ICU acquired hypermagnesemia among those with normal (odds ratio; OR, 1.34; 95% CI, 1.02−1.77; p = 0.034) and low (OR, 1.67; 95% CI, 1.15−2.41; p = 0.006) admission magnesium levels increased the risk for death. Magnesium abnormalities are common among patients managed in ICUs. The determinants of ICU-acquired hypermagnesemia and its adverse effect on outcome warrants further investigation.