This case report features a 62-year-old male with stage IB lung adenocarcinoma harboring an epidermal growth factor receptor exon 19 deletion, who underwent treatment with osimertinib following a left upper lobectomy and lymph node dissection. Despite a history of smoking and well-managed type 2 diabetes, the patient developed heart failure 18 months post-initiation of osimertinib therapy, marking one of the latest occurrences of heart failure following osimertinib treatment documented in limited literature. Cardiac MRI revealed significant left ventricular enlargement, lateral wall myocardial thinning, and localized myocardial fibrosis without perfusion defects, a finding not previously reported in literature. This case underscores the severe and unusual cardiac effects of osimertinib in patients with latent risk factors, highlighting the importance of vigilant cardiac monitoring and a multidisciplinary management approach.