Background: Our ability to optimize the care of older adults with cancer and comorbid illnesses is insufficient because most clinical trials lack systematic measurement. The primary purpose of this study was to evaluate the association between patient-reported comorbidity and all-cause mortality using various comorbidity scoring algorithms. Materials and Methods: The Carolina Senior Registry was linked with the North Carolina Central Cancer Registry to obtain mortality data. Comorbidity was assessed using the patient-reported Older Americans Resources and Services Questionnaire subscale that assesses 13 specific conditions and the degree to which each impairs activities. Multivariable Cox proportional hazard regression models were used to evaluate the association between comorbidities and all-cause mortality. Results: The study sample included 539 patients; the median age was 72 years, 72% were female, and 47% had breast cancer. Overall, 92% reported ≥1 comorbid condition, with a mean of 2.7 conditions (range 0–10), with arthritis and hypertension the most common (52% and 50%, respectively). Approximately 60% reported a functional limitation related to comorbidity. After adjusting for time from diagnosis to geriatric assessment, age, cancer type, and stage, the risk of death increased by 5% for each unit increase in comorbidity burden score (adjusted hazard ratio [HR] = 1.05, 95% confidence interval [CI]: 1.01–1.10) and 12% for each comorbid condition impacting function (HR = 1.12, 95% CI: 1.02–1.23). Conclusion: Comorbid conditions in older adults with cancer are highly prevalent and associated with all-cause mortality, particularly those conditions that impair function. Routine comorbidity assessment should be included in clinical trials and can be measured via a simple one-page patient-reported questionnaire. Implications for Practice: In order to optimize and personalize the care of older adults with cancer, systematic measurement of comorbidities is necessary in both clinical trials and routine practice. Patient-reported comorbid conditions in older adults with cancer are highly prevalent and are associated with increased risk of all-cause mortality, particularly for those conditions that impair function. Comorbidity can be systematically measured via a one-page patient-reported questionnaire and should be incorporated into future clinical trials and considered for use in oncology clinics to aid in assessing older adults with cancer.