Background Colostomy reversal is a common procedure. Patients often have baseline comorbidities associated with postoperative morbidity. We utilized a modified frailty index (mFI-5) to predict postoperative complications. Methods Patients who underwent elective, open Hartmann’s reversal were queried from the National Surgical Quality Improvement Program (NSQIP) database. Patients were stratified to low, medium, or high frailty groups. Statistical analysis was performed using chi-squared, ANOVA, and logistic regression. Results There were 9272 patients with Hartmann’s reversal. 48.78%, 30.31%, and 12.89% had low, moderate, or high frailty, respectively. High frailty was associated with cardiac arrest, myocardial infarction, reintubation, prolonged intubation, early reoperation, and mortality. After multivariate analysis, high frailty was associated with prolonged intubation (OR 3.147, P = .001), reintubation (OR 2.548, P = .002), and reoperation (OR 1.67, P < .001). Conclusions Frailty was associated with greater risk of postoperative complications in patients undergoing Hartmann’s reversal. Frailty may be a useful adjunct to stratify for patients who are at risk for postoperative complications.