Purpose: Magnesium is frequently measured and administered in general intensive care unit patients. However, magnesium status, its association with outcomes, and therapeutic utility in such patients are unclear. We performed a systematic review of the relevant literature to define current knowledge in this field. Materials and Methods: We searched MEDLINE, CENTRAL, and EMBASE from 1975 to July 2014 for adult English language articles excluding obstetric, non-intensive care unit based, and specific population (poisoning, cardiothoracic, and neurosurgery) studies. We identified articles on magnesium measurement, associations, and therapy. We calculated pooled effect estimates from reported adjusted risk estimates. Results: We identified 34 relevant studies. Total serum total magnesium was the most commonly measure of magnesium status. Risk of mortality was significantly increased with hypomagnesemia (odds ratio, 1.85; 95 confidence interval, 1.31-2.60). No consistent associations existed between magnesemia or magnesium administration and any other outcomes. Conclusions: Total serum magnesium levels are generally used to estimate magnesium status in critical illness. Hypomagnesemia appears associated with greater risk of mortality, but the efficacy of magnesium administration is open to challenge.