Background: The objectives of this study were to determine whether frailty is associated with survival in a population-based sample of patients with diffuse large B-cell lymphoma (DLBCL) and to describe the healthcare utilization patterns of frail versus nonfrail patients during treatment. Methods: A retrospective cohort study was conducted using population-based data in Ontario, Canada. Patients aged ≥66 years diagnosed between 2006 and 2017 with DLBCL or transformed follicular lymphoma who received first-line curative-intent chemoimmunotherapy were included. Frailty was defined using a modified version of a generalizable frailty index developed for use with Ontario administrative data. Cox regression was performed to examine the association between frailty and 1-year mortality. Results: A total of 5,527 patients were included (median age, 75 years [interquartile range, 70–80 years]; 48% female), of whom 2,699 (49%) were classified as frail. Within 1 year of first-line treatment, 32% (n=868) of frail patients had died compared with 20% (n=553) of nonfrail patients (unadjusted hazard ratio, 1.8; 95% CI, 1.6–2.0; P<.0001). Frail patients had higher healthcare utilization during treatment, with most hospitalizations related to infection and/or lymphoma. In multivariable modeling controlling for age, inpatient diagnosis, number of chemoimmunotherapy cycles received, comorbidity burden, and healthcare utilization, frailty remained independently associated with 1-year mortality (adjusted hazard ratio, 1.5; 95% CI, 1.3–1.7; P<.0001). Conclusions: In a population-based sample of older adult patients with DLBCL receiving front-line curative-intent therapy, half were classified as frail, and their adjusted relative rate of death in the first year after starting treatment was 50% higher than that of nonfrail patients. Frailty seems to be associated with poor treatment tolerance and a higher likelihood of requiring acute hospital-based care.