Background: Previous studies found that low levels of magnesium can increase the risk of several diseases including cardiovascular diseases, diabetes, and hypertension which are associated with cognitive dysfunction. Objective: Examine the association between serum magnesium and cognitive function among Qatari adults. In addition to assessing the interaction relation between low serum magnesium, hypertension, and diabetes in relation to cognitive function. Methods: Data from 1000 Qatari participants aged ≥20 years attending the Qatar Biobank Study were analyzed. Serum magnesium was measured by an automated calorimetric method and suboptimal magnesium was characterized by <0.85 mmol/L. Cognitive function was measured by a computer-based selfadministered test focusing on mean reaction time (MRT). Multivariable linear regression and logistic regression were used. Results: The prevalence of suboptimal magnesium was 57.1%. Across the quartiles of serum magnesium from high to low, the regression coefficients (95% CI) for MRT were 0, -17.79, -18.27, and -31.93 (95%CI 2.38–3.05), respectively (p for trend <0.033). The presence of hypertension and diabetes significantly increased the MRT along with a wide range of low serum magnesium. Women with diabetes or hypertension were affected the greatest by low magnesium levels. Conclusion: There was a positive association between serum magnesium and cognitive function. Low magnesium levels were associated with a longer MRT.