Magnesium and calcium are elements that have been associated with cardiometabolic risk factors related to metabolic syndrome (MetS). However, there are gaps in the knowledge regarding the impact of the calcium to magnesium (Ca/Mg) ratio in plasma. Thus, we aim to evaluate the associations between magnesium and calcium levels in plasma, and the Ca/Mg ratio in plasma with MetS components and other cardiometabolic risk factors. This cross-sectional study was carried out with 112 adults and older people, distributed into groups with (n = 60) and without MetS (n = 52). We evaluated sociodemographic, anthropometric, and biochemical data. Magnesium and calcium levels in plasma were measured by inductively coupled plasma mass spectrometry technique (ICP-MS). There was a high frequency of MetS, with no significant differences in magnesium and calcium levels and Ca/Mg ratio in plasma observed between groups. There were no associations between magnesium and MetS components or other cardiometabolic risk factors (all p > 0.05). Calcium levels were associated with total cholesterol (β =  - 0.020; p = 0.000) and high-density lipoprotein cholesterol (HDL-c) (β =  - 0.046; p = 0.005). The total cholesterol (β =  - 0.025; p = 0.000) and low-density lipoprotein cholesterol (LDL-c) (β = 0.017; p = 0.020) were preditors of the Ca/Mg ratio. These results indicate important associations of calcium and the Ca/Mg ratio in plasma with cardiometabolic risk factors related to MetS.