A 6-year-old female spayed Domestic Shorthair cat presented to the Cornell University for Animal’s Soft Tissue Surgery service for evaluation and treatment of a previously diagnosed mammary adenocarcinoma. Approximately one month prior to presentation, the owner had noticed a single small mass on the left mammary chain. An excisional biopsy was performed by the referring veterinarian and sent for histopathologic analysis. Results revealed a “simple mammary gland adenocarcinoma, tubular and solid, intermediate grade with vascular invasion.” The margins were 1 mm and appeared to be completely excised. A chain mastectomy of the left side was performed by the Soft Tissue Surgery service without complications. The patient was set to return six to eight weeks later for the right chain mastectomy. Mammary adenocarcinoma in feline patients has a very high metastatic rate. Due to its nature, few treatment options exist. Diagnosis, treatment, and prognosis of feline mammary adenocarcinoma will be discussed below.