Background: Cancer and cardiovascular disease (CVD) are independently associated with adverse outcomes in patients with COVID-19. However, outcomes in patients with COVID-19 with both cancer and comorbid CVD are unknown. Methods: This retrospective study included 2,476 patients who tested positive for SARS-CoV-2 at 4 Massachusetts hospitals between March 11 and May 21, 2020. Patients were stratified by a history of either cancer (n=195) or CVD (n=414) and subsequently by the presence of both cancer and CVD (n=82). We compared outcomes between patients with and without cancer and patients with both cancer and CVD compared with patients with either condition alone. The primary endpoint was COVID-19–associated severe disease, defined as a composite of the need for mechanical ventilation, shock, or death. Secondary endpoints included death, shock, need for mechanical ventilation, need for supplemental oxygen, arrhythmia, venous thromboembolism, encephalopathy, abnormal troponin level, and length of stay. Results: Multivariable analysis identified cancer as an independent predictor of COVID-19–associated severe disease among all infected patients. Patients with cancer were more likely to develop COVID-19–associated severe disease than were those without cancer (hazard ratio [HR], 2.02; 95% CI, 1.53–2.68; P<.001). Furthermore, patients with both cancer and CVD had a higher likelihood of COVID-19–associated severe disease compared with those with either cancer (HR, 1.86; 95% CI, 1.11–3.10; P=.02) or CVD (HR, 1.79; 95% CI, 1.21–2.66; P=.004) alone. Patients died more frequently if they had both cancer and CVD compared with either cancer (35% vs 17%; P=.004) or CVD (35% vs 21%; P=.009) alone. Arrhythmias and encephalopathy were also more frequent in patients with both cancer and CVD compared with those with cancer alone. Conclusions: Patients with a history of both cancer and CVD are at significantly higher risk of experiencing COVID-19–associated adverse outcomes. Aggressive public health measures are needed to mitigate the risks of COVID-19 infection in this vulnerable patient population.