Background: Many elderly neurological disorders are associated with phosphate, magnesium, and calcium deficiency. Objective: To assess the correlation between cognitive dysfunction and serum magnesium, calcium and phosphorus level in the elderly Egyptian patients. Patients and Methods: From July 2021 to October 2021, at Zagazig University Hospitals inpatient and outpatient clinics of Internal Medicine Department, our case control trial was conducted on 94 elderly patients with and without impaired cognitive function, they were divided into two equal groups (47 each group). All patients underwent a laboratory investigation to assess serum levels of magnesium, calcium, and phosphorus. Assessment of cognitive function was done through answering a series of questions and/or performing simple tasks. Results: There was highly statistically significant correlation between the two studied groups (normal and abnormal cognitive functions) and serum calcium, magnesium, and phosphorus levels (P <0.001 for the 3 correlations), there was statistically significant correlation between Mg²⁺ and Montreal Cognitive Assessment (MoCA) test, Mini mental state examination and Recall Mini-COG test in abnormal group, there was statistically significant correlation between P³⁺ and age in abnormal cognitive functions group, MoCA test, Mini mental state examination and Recall Mini-COG test in abnormal group. Conclusion: Calcium and magnesium are associated with cognitive improvement in elderly, but in contrast, higher level of phosphorus facilitates cognitive decline. We concluded that age, magnesium and phosphorus were independent variable for cognitive impairment.