We report a case of severe mitral stenosis caused by Libman-Sacks endocarditis, as an initial manifestation of systemic lupus erythematosus (SLE) in a 20-year-old woman. Cardiac magnetic resonance imaging (MRI) demonstrated a thickening of the mitral valve with basal endocardial thickening exhibiting defect on first-pass perfusion short-axis acquisition and delayed enhancement in keeping with extensive fibrous endocarditis. The patient underwent successful mechanical mitral valve replacement. This case illustrates that MRI is useful in diagnosing this recognised but uncommon cardiac complication of SLE and excluding differential diagnosis such as valve tumour and infective endocarditis with perivalvular abscesses.