Magnesium sulfate is used as prophylaxis and treatment of severe preeclampsia/eclampsia, albeit its safety and toxicity are a concern. We designed this study to estimate the incidence of critical hypermagnesemia in severely preeclamptic women under a magnesium sulfate regimen at 8 h following its administration and to identify the associated risk factors as the primary outcomes. Also, secondary outcomes were to compare baseline characteristics, laboratory findings, and maternal-neonatal complications stratified by the baseline serum magnesium (Mg2+) in those women, and to assess the degree of agreement between patellar reflex and serum Mg2+ concentration 8 h following magnesium sulfate administration. We conducted a retrospective study including severely preeclamptic women receiving magnesium sulfate from June 2016 to May 2021. We enrolled 429 women in the study. Two-hundred sixty-one (60.8%) of the included women developed critical hypermagnesemia. Preeclamptic women with high baseline serum Mg2+ concentration demonstrated significantly affected renal functions, hepatic transaminase activities, and low platelet count as well as more reported maternal complications compared to those with low baseline serum Mg2. Multivariable logistic regression revealed that a lower gestational age, a higher uric acid concentration, and a higher baseline serum Mg2+ concentration were independently associated with an increased risk of critical hypermagnesemia. The agreement between deep tendon reflex assessment and serum Mg2+ concentration was slight although not significant. The maternal-neonatal outcomes were non-significant in women with critical hypermagnesemia. More vigilant monitoring through assessment of both serum Mg2+ concentration and deep tendon reflex should be considered especially in high-risk women.