Li-Fraumeni syndrome (LFS) is one of the most penetrant forms of familial cancer susceptibility syndromes, characterized by early age at tumor onset and a wide spectrum of malignant tumors. Identifying LFS in patients with cancer is clinically imperative because they have an increased sensitivity to ionizing radiation and are more likely to develop radiation-induced secondary malignancies. This case report describes a young woman whose initial presentation of LFS was early-onset breast cancer and whose treatment of this primary malignancy with breast conservation likely resulted in a secondary malignancy arising in her radiation field. As seen in this case, most breast cancers in patients with LFS exhibit a triple-positive phenotype (estrogen receptor–positive/progesterone receptor–positive/HER2-positive). Although this patient met classic LFS criteria based on age and personal and family history of cancer, the NCCN Clinical Practice Guidelines in Oncology for Genetic/Familial High-Risk Assessment: Breast and Ovarian Cancer endorse genetic screening for TP53 mutations in a subset of patients with early-onset breast cancer, even in the absence of a suggestive family history, because of the potential for de novo TP53 mutations.