Background: Mounting evidence suggests that dermatomyositis/polymyositis (DM/PM) are associated with increased risk of atherosclerotic events and venous thromboembolism. However, data on the association between DM/PM and other cardiac outcomes, especially heart failure (HF), are scarce. Objectives: To examine the long-term risk and prognosis associated with adverse cardiac outcomes in patients with DM/PM. Methods: Using Danish administrative registries, we included all patients ≥18 years with newly diagnosed DM/PM (1996–2018). Risks of incident outcomes were compared with non-DM/PM controls from the background population (matched 1:4 by age, sex, and comorbidity). In a secondary analysis, we compared mortality following HF diagnosis between DM/PM patients with HF and non-DM/PM patients with HF (matched 1:4 by age and sex). Results: The study population included 936 DM/PM patients (median age 58.5 years, 59.0% women) and 3744 matched non-DM/PM controls. The median follow-up was 6.9 years. Absolute 10-year risks of incident outcomes for DM/PM patients vs matched controls were as follows: HF, 6.98% (CI, 5.16–9.16%) vs 4.58% (3.79–5.47%) (P = 0.002); atrial fibrillation, 10.17% (7.94–12.71%) vs 7.07% (6.09–8.15%) (P = 0.005); the composite of ICD implantation/ventricular arrhythmias/cardiac arrest, 1.99% (1.12–3.27%) vs 0.64% (0.40–0.98%) (P = 0.02); and all-cause mortality, 35.42% (31.64–39.21%) vs 16.57% (15.10–18.10%) (P < 0.0001). DM/PM with subsequent HF was associated with higher mortality compared with HF without DM/PM (adjusted hazard ratio 1.58 [CI, 1.01–2.47]). Conclusion: Patients with DM/PM had a higher associated risk of HF and other adverse cardiac outcomes compared with matched controls. Among patients developing HF, a history of DM/PM was associated with higher mortality.