Background: Patients with idiopathic inflammatory myopathies (IIM) have an increased risk of cancer, but their cancer-related disease burden remains unclear. Objectives: To explore how cancer might impact the mortality of patients with IIM and examine the associated prognostic factors for cancer and death. Methods: We identified patients with IIM diagnosed between 1998 and 2020 and ascertained their cancer and death records via linkage to the Swedish healthcare and population registers. Transition hazards from IIM diagnosis to cancer and death were estimated in multistate models using flexible parametric methods. We then predicted the probability of having cancer or death, and the duration of staying alive at a given time from IIM and cancer diagnoses from a crude model. We also explored prognostic factors for progression to cancer and death in a multivariable model. Results: Of 1826 IIM patients, 310 (17%) were diagnosed with cancer before and 306 (17%) after IIM diagnosis. In patients diagnosed with cancer after IIM, the 5-year probability of death from cancer and from other causes was 31% and 18%, respectively, compared to 7% and 15% in patients without cancer after IIM. We reported several factors associated with risk of progression to cancer and death. Specifically, patients with first cancer after IIM who were older at IIM diagnosis, had cancer history, dermatomyositis and a cancer diagnosis within 1 year following IIM faced a greater cancer-specific mortality. Conclusion: We observed a substantial increase in mortality from cancer, compared to before, rather than other causes after a cancer diagnosis following IIM, suggesting an unmet medical need for effective cancer management in IIM patients. This finding, along with the identified prognostic factors, provides useful insight into future research directions for improving cancer management in IIM patients.