Combined administration of two or more drugs is emerging as a new strategy in triple-negative breast neoplasms. This is the first study to investigate the combination of the histone deacetylase inhibitor mocetinostat and the antimetabolite drug capecitabine in triple-negative mammary neoplasms in a preclinical mouse model. Thirty-five female mice were grouped into the control group, capecitabine group, mocetinostat group, and combined drugs group. At the end of the experimental period, body weight, and tumor weight were measured and tumor tissue and lung tissue were histologically examined. The results showed that the body weight of mice in the drug-treated groups was reduced by about 18%. Tumor weights were also reduced by 21% in the mocetinostat group, 27.5% in the capecitabine group, and 45% in the combined group. The combination of mocetinostat and capecitabine decreased the formation of tumors and metastases in lung tissue. In summary, the combination of mocetinostat and capecitabine was more effective than either drug alone in reducing the size of triple-negative breast neoplasms in a mouse model.