In the present study, we assessed the effects of patient frailty status on the early outcomes and late survival after fenestrated-branched endovascular aortic repair (FB-EVAR) for complex abdominal and thoracoabdominal aortic aneurysms. We retrospectively reviewed the clinical data and outcomes of consecutive patients who had undergone elective FB-EVAR from 2007 to 2019 in a single institution. A previously validated 11-item modified frailty index (mFI-11) was derived from the comorbidity and preoperative functional status data. An mFI-11<0.3 was defined as low risk, 0.3 to 0.5 as medium risk, and >0.5 as high risk. The studied outcomes were 90-day mortality, major adverse events (MAE), and long-term survival. Multivariate analyses were performed to identify the independent predictors of these outcomes. A total of 592 patients (155 women, mean age, 75± 8years) had undergone FB-EVAR. Using the mFI-11, 310 patients (52%) were included in the low-risk, 199 (34%) in the medium-risk, and 83 (14%) in the high-risk group. The 90-day mortality was significantly higher in the high-risk group than in the medium- and low-risk groups (13%, 4%, and 3%, respectively; P< .01). The corresponding MAE rates were 27%, 18%, and 19% (P= .23). As a subgroup, 44 patients in the high-risk group had had chronic kidney disease (CKD). The 90-day mortality for these patients was as high as 23%, and 32% had experienced MAE. On multivariable analysis, the independent risk factors for 90-day mortality were CKD, respiratory disease, and a high mFI-11. The independent risk factors for MAE were female sex, CKD, larger aneurysm diameter, and the high-risk subgroup with CKD. The independent risk factors for long-term mortality were age, a low body mass index, CKD, larger aneurysm diameter, extent I-III thoracoabdominal aortic aneurysm, respiratory disease, congestive heart failure, a history of cerebrovascular problems, and higher mFI-11. The estimated survival at 1year was 91%± 2% in the low-risk, 88%± 2% in the medium-risk, and 78%± 5% in the high-risk group (P< .001). The corresponding 5-year survival estimates were 60%± 4%, 52%± 5%, and 32%± 6%. The mean follow-up time was 2.9± 2.3years. The patients treated during the first quartile of the study period were significantly more frail than were those in the later quartiles. Also, the outcomes of FB-EVAR had improved over time. Greater frailty was significantly associated with early mortality. Together with CKD, frailty was also associated with MAE and lower patient survival after FB-EVAR. The mFI-11 represents the accumulation of comorbidities and can be used to assist in better patient selection for FB-EVAR.