The aim of this study was to investigate cardiac involvement in patients with idiopathic inflammatory myopathies excluding inclusion body myositis with cardiac magnetic resonance tomography (CMR). A case series of 53 patients with polymyositis, dermatomyositis, or non-specific myositis underwent CMR including functional imaging, T1-weighted, and late gadolinium enhancement (LGE) imaging. T1-weighted and LGE images were analyzed for myocardial enhancement. Reduced left ventricular function (LVF) was found in 9 (7%) patients. Patients with reduced LVF more often presented with early and late myocardial enhancement (p = 0.014 and p = 0.001). In 33 (62.3%) patients, LGE was observed by CMR. These patients had significantly lower left ventricular ejection fractions (p < 0.001) compared to patients without LGE. LGE was mainly present in the lateral (p < 0.01) and inferior (p < 0.02) segments. No correlations of LGE presence or reduced LVF to cardiovascular risk factors were found. Myocardial inflammation is very frequent in polymyositis, dermatomyositis, and non-specific myositis. In our patient, cohort CMR demonstrated signs of myocardial inflammation in 62.3%. CMR seems to offer a measurable and quantifiable diagnostic tool for cardiac involvement of idiopathic inflammatory myopathies and can thus be used to monitor disease progress and therapeutic success in these patients.