BACKGROUND The decision to emergently operate on nonagenarian patients (NONAs) can be complex due to the uncertainty about outcomes and goals of care at this advanced age. We sought to study: (1) the outcomes and predictors of mortality for NONAs undergoing emergency general surgery (EGS) and (2) the accuracy of ACS-NSQIP mortality risk calculator in this special population. METHODS Using the 2007 to 2015 ACS-NSQIP database, we included all patients older than 90 years of age who underwent an emergent operation with a Current Procedural Terminology (CPT) code for "digestive system." Multivariable logistic regression analyses were performed to identify independent predictors of 30-day mortality. NONAs' mortality rates for different combinations of risk factors were also studied and compared to the ACS-NSQIP calculator-predicted mortality rates. RESULTS Out of a total of 4,456,809 patients, 4,724 NONAs were included. The overall 30-day patient mortality and morbidity rates were 21% and 45%, respectively. In multivariable analyses, several independent predictors of 30-day mortality were identified, including recent history of weight loss, history of steroid use, smoking, functional dependence, hypoalbuminemia and sepsis or septic shock. The mortality among NONAs with a history of steroid use and a recent history of weight loss was 100%. Similarly, the mortality of NONAs with recent history of weight loss who presented with preoperative septic shock was 93%. The ACS-NSQIP calculator significantly and consistently underestimated the risk of mortality in all NONAs undergoing EGS. CONCLUSION Most NONAs undergoing EGS survive the hospital stay and the first 30 postoperative days, even in the presence of significant preexisting comorbidities. However, the combination of recent weight loss with either steroid use or septic shock nearly ensures mortality and should be used in the discussions with patients and families before a decision to operate is made. The ACS-NSQIP surgical risk calculator should be used with caution in these high-risk patients.