Background: A significant proportion of patients presenting for isolated aortic valve replacement (AVR) demonstrate some degree of functional mitral regurgitation (fMR). Guidelines addressing concomitant mitral valve intervention in those patients with moderate fMR lack strong evidence-based support. Our aim is to determine the effect of untreated moderate fMR at the time of AVR on long-term survival. Methods: All patients undergoing isolated AVR from 2000 to 2013 at our institution were retrospectively reviewed. Patients were stratified according to severity of preoperative fMR; 0-1+ MR (Group NoMR, n = 1826) and 2-3+ MR (Group MR, n = 330). All patients in Group MR were propensity-matched with patients in Group NoMR to control for differences in baseline characteristics. The primary outcome of interest was overall survival. Results: Propensity analysis matched 330 patients from each group. Mean age was 77.9 ± 10.0 years and 50.6% were male. There were no differences in baseline demographics, echocardiographic parameters, or co-morbidities between groups. Kaplan-Meier analysis showed significantly worse medium and long-term survival in Group MR compared to Group NoMR (log-rank p = 0.02). Follow-up echocardiography showed slightly more severe MR in Group MR (1.1 ± 0.7 MR vs. 0.8 ± 0.7 NoMR, p = 0.03) at 1 year. Conclusions: Patients undergoing isolated AVR with concomitant 2-3+ fMR experience poorer long-term survival than those patients with no or mild fMR. This suggests that mitral valve intervention may be necessary in patients undergoing AVR with clinically significant fMR.