Background: Mucinous cystadenocarcinoma (MCA) mainly occurs in the ovary, pancreas, and appendix, whereas the breast is a rare primary site of occurrence. Invasive ductal carcinoma (IDC) is the most common breast malignancy. Only 31 cases of the breast MCA have been reported in the English literature, and the coexistence of MCA and IDC in the breast are rare.Case Description: Here, we describe a 61-year-old postmenopausal woman with no family history of breast cancer or other breast-related diseases, who presented with a palpable mass in her left breast lasting for 2 months. On ultrasonography examination, the tumor was a cystic-solid lesion with clear boundary. Magnetic resonance imaging (MRI) showed a mass with low signal intensity on T1 weighted imaging and high signal intensity on T2 weighted imaging. Intraoperative frozen sections revealed metastatic tumor cells in one sentinel lymph node (1/4). She then underwent left modified radical mastectomy with axillary dissection. The post-operative pathological examination showed the tumor consisted mostly of MCA (60%), with a small proportion of intermediate-grade IDC. The MCA had a well-demarcated cystic structure with papillary projections and abundant mucoid material. The epithelium lining cystic spaces was tall columnar, with mucin-producing cells that had basally located nuclei. The degree of cytological atypia varied considerably. Axillary lymph nodes were normal (0/15). The MCA was triple-negative for estrogen receptor (ER), progesterone receptor (PR), and HER2, and positive for CK7 but negative for CK20. Through next generation sequencing, no mutations in the BRCA1 and BRCA2 genes were identified in our case, which was not highlighted in prior cases. After surgery, the patient underwent eight cycles of chemotherapy, and she has been disease-free during the 10-month follow-up. In addition to detailing this instance of mixed MCA and IDC of the breast, we reviewed relevant literature and compare our findings with other patients who had breast MCAs.Conclusions: Our results improved the understanding of mixed MCA and IDC, especially MCA, and provided a basis for its diagnosis and differential diagnosis from other metastatic diseases.