Our study aimed to investigate the association between initial serum magnesium levels and long-term functional outcomes in patients with TBI, accounting for variations across different age groups. This retrospective study included adult patients with TBI and intracranial injury treated at a level-1 trauma hospital. The primary exposure was on serum magnesium levels within 6 hour of emergency department admission. Study outcomes included 6-month mortality and disability, assessed using the Glasgow Outcome Scale (scores 1-3). Multilevel multivariable logistic regression analysis was conducted to determine the relationship between serum magnesium levels and study outcomes, with stratified analysis employed to investigate potential variations based on age groups. Low magnesium levels were significantly associated with an increased risk of 6-month disability (Odds ratio (OR) 1.80, 95% confidence interval (CI): 1.14-2.68) and 6-month mortality (OR: 1.82, 95% CI: 1.03-2.99). This association remained significant exclusively among younger patients, with ORs of 2.70 (95% CI: 1.21-5.14) for 6-month disability and 2.33 (95%, CI: 1.11-4.49) for 6-month mortality when stratified by age. Low serum magnesium levels are associated with poorer long-term outcomes in patients with TBI, and this effect is especially pronounced in younger adults (18-64 years). Specifically, younger patients exhibiting hypomagnesemia demonstrated a higher incidence of 6-month disability and mortality. These findings suggest a need to identify the causative links and explore whether correcting serum magnesium levels will improve TBI prognosis across different age groups.