Magnesium is an essential mineral in human metabolism, and is the second most abundant intracellular cation and the fourth most abundant in the human body. Magnesium has a large variety of biological functions, including being a cofactor for over 300 enzymes, mainly involving phosphorylation of proteins and nucleic acids. Low levels of magnesium in the body can develop during different illnesses, such as diabetes (type 2), metabolic syndrome, cardiac arrhythmias, and muscular constriction. The present review shows the advances in analytical chemistry based on sensor systems to quantify the concentration of magnesium in different biological samples, plasma, serum, or urine, that are relevant to human health.