Multi-modality imaging is the recommended approach to assess the severity of valvular heart diseases. Rheumatic mitral stenosis (MS), however, has yet to benefit from this approach. The aim of this study is to assess the added value of cardiac MRI (CMR) and computed tomography (CT) calcium score in assessing severity of MS when compared to 3D echocardiography. Patients with MS in sinus rhythm were included. Both CMR and 3D echo assessments of the mitral valve were performed. Subsequently, three radiologists and three cardiologists independently measured mitral valve area (MVA). In addition, CT of the mitral valve was conducted. Mitral calcium score was calculated according the Agatston method. A total of 41 patients were included. CMR significantly overestimated MVA when compared to 3D echo MVA regardless of the investigator [F (1, 40) = 23.3, p < 0.001, η2 = 0.36]. The more severe the MS, the greater the overestimation by CMR compared to 3D echo. Regarding CT of the mitral valve, 25 (61%) patients had an undetectable calcium. There was no significant difference in CT calcium scores between severe and non-severe MS (74 ± 282 HU vs. 65 ± 210 HU, p = 0.9). MVA measurement by CMR is overestimated when compared to 3D echo. Additionally, mitral valve calcium score is not correlated to MS severity.