Background: While multimorbidity clustering is a significant problem in older adults, the impact of clusters present prior to cancer on post-diagnosis survival and function is unknown. We used SEER-Medicare Health Outcomes Survey data for 4583 cancer patients to address this research gap. Methods: Patients with prostate (1741), breast (BC: 1345), colorectal (CRC: 904) and lung (593) cancer with pre- and post-diagnosis survey data were included. Surveys assessed comorbidity and activities of daily living (ADLs). Previously defined multimorbidity clusters were cardiovascular disease (CVD), skeletal, metabolic, pulmonary + major depressive disorder (MDD), and gastrointestinal (GI) + MDD. Cox regression models estimated hazard ratios (HR) for death after cancer diagnosis. Among those without pre-cancer ADL impairment, modified Poisson regression models estimated relative risk (RR) for developing post-cancer functional impairment (ADL ≤ 4). Models controlled for age, race, education, poverty level, stage, and treatment (radiation, surgery). Results: Median age at cancer diagnosis was 74y (65-103). Post-diagnosis mortality: After 6y median follow-up, mortality was 30%; 5y survival was 74%.Prostate, BC and CRC patients with pre-diagnosis CVD clusters were at increased risk of death compared to those without CVD cluster (HR 1.9, 2.0, 1.7, respectively, p < 0.05). Compared to those without the cluster, prostate and BC patients with metabolic cluster were at increased risk (HR 1.7, 1.9, respectively, p < 0.05) and prostate cancer patients with pulmonary conditions + MDD or GI + MDD (HR 1.9, 2.1, respectively, p < 0.05) were at increased risk. Post-diagnosis functional impairment: Prevalence of moderate functional impairment at a median of 1y after cancer diagnosis was 31%. Prostate, lung, and CRC survivors with GI + MDD had a significant RR of developing impairment (RR 1.8, 1.8, and 1.7, p < 0.001). For BC patients, those with skeletal cluster had a 2.1 RR (p < 0.001). Conclusions: Specific multimorbidity clusters prior to cancer are associated with post-cancer mortality and ADL impairment and identify at-risk groups where interventions can be instituted to decrease morbidity and mortality.