Background: Frailty has been studied extensively in trauma, but there is minimal research detailing its impact on traumatic brain injury (TBI). We hypothesized that the 11-item modified frailty index (mFI-11) would predict complications and discharge outcomes in patients with TBI. Methods: A retrospective review of our trauma quality improvement program (TQIP) registry was conducted for all patients with TBI. The mFI-11 score was calculated for each patient. Multivariable logistic regression was used to assess the relationship between mFI-11 and cardiovascular, infectious, pulmonary, renal, thromboembolic, and unplanned complications (ie, unplanned intensive care unit [ICU] admission, intubation, or return to the operating room). Results: There were 2352 patients with TBI of whom 61.6% (n = 1450) were not frail, 19.3% (n = 454) were mildly frail, and 19.1% (n = 448) were moderately to severely frail. Higher frailty scores were associated with increasing age (P <.0001) and decreasing injury severity score [ISS] (P = 0.001). Higher frailty scores also correlated with increasing rates of a skilled nursing facility/long-term acute care hospital/rehabilitation discharge (P =.0002). On multivariable logistic regression adjusting for age, Glasgow Coma Scale (GCS) score, ISS, mechanism, and sex, moderate to severe frailty increased the odds of acute kidney injury (odds ratio [OR] 2.06, 95% CI 1.07-3.99, P =.03) and any unplanned event (OR 1.6, 95% CI 1.1-2.3, P =.01). Conclusion: Frailty measured by the mFI-11 is associated with greater rates of discharge to unfavorable locations and increased odds of acute kidney injury and unplanned events among patients with TBI. These findings suggest that frail patients with TBIs require greater vigilance to avoid such unanticipated outcomes.