Background and objectives: A study of prevalence of Hypomagnesemia in critically ill patients of all age group and it’s significance in mortality among them. Materials and methods: 100 cases admitted at Intensive Medical care unit of Government Kilpauk Medical college Hospital, Chennai, during the period April 2015 to September 2016, from various causes, were included in the study after taking into account the inclusion and exclusion criteria. Out of the 100 cases 56 were males and 44 were females. All the 100 cases were treated for relevant diseases and their laboratory work up and follow up data were collected and studied. Results: In our study we analysed 100 patients, in that 53 persons are hypomagnesemics, 47 persons are normomagnesemics. The minimum age of the patient is 18. The maximum age is 86. Among 100 population, magnesium levels was found to be low in 16 cases of sepsis, 9 cases of CVA, 7 cases of poisoning, 6 cases of heart failure, 5 cases of respiratory failure, 4 cases of DKA, 3 cases of status epilepticus and 3 cases of snake bite. Among these, patients with sepsis had more percentage of hypomagnesemic levels. Of 100 patients 20 died, out of which 16 had hypomagnesemia. P<0.05 which is statistically significant. Conclusion: Hypomagnesemia is a common electrolyte imbalance in the critically ill patients, frequently associated with sepsis and associated with higher mortality and morbidity rate. Correction of hypomagnesemia is very essential in the Management of critically ill patients to have a better prognosis.