Magnesium (Mg) is the second most common intracellular electrolyte; in the body. Few data are available in general population studies on the assessment of Mg status and its relationship to dietary intake. We explored the relationship between several biological Mg surrogates (serum, red cell, and urinary), Mg intake, age, alcohol intake, and mean energy intake in a large sample of French adults issued from the SUVIMAX study. Serum Mg was positively correlated with red cell Mg in men and women (r = 0.17; p < 0.001) whereas no association was found between urinary Mg and red cell Mg nor between urinary Mg and serum Mg. Age was positively correlated with serum and red cell Mg in women (p < 0.001) and negatively correlated with urinary Mg in both genders (p < 0.001). Finally, a negative correlation was found between dietary intake and red cell Mg in women (r = -0.06; p < 0.05). Among the biological Mg surrogates, an association was found between serum Mg and red cell Mg. Further investigations should be conducted in order to determine the role played by dietary Mg intake in the relationship between health status and several biological Mg measurements.