Giant left atrium (GLA) is a well-described entity in association with rheumatic heart disease. However, mitral valve prolapse is an extremely unusual cause of GLA, especially without the compressive symptoms it can often accompany. We discuss a case of a 78-year-old lady with no prior history of rheumatic heart disease with these findings with the unusual presentation of accompanied unilateral breast oedema as a manifestation of heart failure. This case illustrates the investigations and treatment options for GLA and the need for prompt assessment as it increases the risk of sudden death; therefore, its presence warrants careful evaluation and surgical intervention when appropriate.