Background: Monoclonal antibody (mAb) treatment may prevent complications of COVID-19. We sought to quantify the impact of bamlanivimab monotherapy on hospitalizations and mortality, as well as Emergency Department (ED) visits without hospitalization, among outpatients at high risk of COVID-19 complications. Methods: We compared patients receiving mAb to patients who met criteria but did not receive mAb from December 2020 through March 2021. The study population selection used propensity scores to match 1:1 by likelihood to receive mAb. The primary outcome was hospitalization or all-cause mortality within 28 days; the secondary outcome was hospitalization or ED visit without hospitalization within 28 days. Odds ratios (OR) calculation used logistic regression modeling including propensity score and mAb receipt predictors. Results: The study population included 234 patients receiving mAb and 234 matched comparator patients not receiving mAb. Patients receiving mAb were less likely to experience hospitalization or mortality (OR 0.31, 95% confidence interval [95%CI] 0.17-0.56, p=0.00001) and hospitalization or ED visit without hospitalization (OR 0.50, 95%CI 0.43-0.83, p=0.007). The impact of mAb was more pronounced in prevention of hospitalization (among all age groups, OR 0.35, 95%CI 0.19-0.66, p=0.001) than mortality or ED visit without hospitalization, and most strongly associated with patients age 65 years and older (primary outcome OR 0.28, 95%CI 0.14-0.56, p=0.0003). Conclusions: Bamlanivimab monotherapy was associated with reduction in the composite outcome of hospitalizations and mortality in patients with mild-moderate COVID-19. The benefit may be strongest in preventing hospitalization in patients ages 65 years or older.