Background: Older cancer patients are at higher risk for poor postoperative outcomes. We tested the validity and utility of Memorial Sloan Kettering-Frailty Index (MSK-FI) in this setting. Methods: In a single institution, prospective cohort study, patients age 75+ received comprehensive geriatric assessment (CGA) by the Geriatrics service during preoperative evaluation. The MSK-FI was developed based on the modified Frailty Index, incorporating 10 comorbid conditions and one item related to basic and instrumental activities of daily living. With the total score ranging from 0 to 11, a score of ≥ 3 was considered frail. We validated the MSK-FI against the CGA, and assessed the relationship between MSK-FI frailty with short term (hospital length of stay (LOS), intensive care unit (ICU) admission) and long-term (overall survival) postoperative outcomes utilizing multivariable linear, logistic, and cox regression models, adjusting for age, duration of surgery, American Society of Anesthesiologists physical status classification, and preoperative albumin level. Results: In total, 1,137 cancer patients (median age 80) were included in the study. The prevalence of frailty based on MSK-FI was 41%. Frail patients were more likely to have poor Karnofsky Performance Status (56% vs. 29%), be dependent in basic and instrumental activities of daily living (72% vs. 40% and 64% vs. 34%), experienced a fall in the past year (30% vs. 18%), have slower gait speed (49% vs. 22%),be depressed (66% vs. 49%), have limited social activity (62% vs. 43%), take ≥ 5 medications (63% vs. 25%), experienced significant weight loss (24% vs. 16%), and suffer from polycomorbid conditions (87% vs. 26%). Frailty was associated with longer LOS in the hospital (1.9 days, p <0.0001) and higher odds of ICU admission (OR = 2.34, p = 0.005). With the median follow up of 12.1 months for survivors, frail patients were at higher risk for overall mortality (HR = 1.67, p < 0.001). Conclusions: MSK-FI is a valid instrument in predicting short and long-term postoperative outcomes of older adults with cancer. Future studies should assess the impact of administering MSK-FI on surgical decision-making, postoperative health care process, and outcomes.