background: the majority of people with cancer have at least one other chronic health condition. with each additional chronic disease, the complexity of their care increases, as does the potential for negative outcomes including premature death. in this paper, we describe cancer patients’ clinical complexity (i.e., multimorbidity; mmb) in order to inform strategic efforts to improve care and outcomes for people with cancer of all types and commonly occurring chronic diseases. methods: we conducted a population-based, retrospective cohort study of adults diagnosed with cancer between 2003 and 2013 (n = 601,331) identified in ontario, canada healthcare administrative data. during a five to 15-year follow-up period (through march 2018), we identified up to 16 co-occurring conditions and patient outcomes for the cohort, including health service utilization and death. results: mmb was extremely common, affecting more than 91% of people with cancer. nearly one quarter (23%) of the population had five or more co-occurring conditions. while we saw no differences in mmb between sexes, mmb prevalence and level increased with age. mmb prevalence and type of co-occurring conditions also varied by cancer type. overall, mmb was associated with higher rates of health service utilization and mortality, regardless of other patient characteristics, and specific conditions differentially impacted these rates. conclusions: people with cancer are likely to have at least one other chronic medical condition and the presence of mmb negatively affects health service utilization and risk of premature death. these findings can help motivate and inform health system advances to improve care quality and outcomes for people with cancer and mmb.