Complete cure of metastatic breast cancer (MBC) is still considered difficult even after the development of new drugs. While new drugs have been continuously developed, conventional drugs such as mitomycin C (MMC) and methotrexate (MTX) have become less used. Combination chemotherapy with MMC and MTX (MMC/MTX) was reported to be effective for 9.7–19.4% of 31 patients with human epidermal growth factor receptor type 2 (HER2)-negative MBC who were aggressively treated with anthracycline, taxane, capecitabine, and vinorelbine. However, its efficacy, when it is used after newly developed drugs such as eribulin and bevacizumab, is yet to be evaluated. We here introduce one case in which MMC/MTX was effective for MBC that was resistant to chemotherapy with eribulin, vinorelbine, and bevacizumab with paclitaxel after sequential treatment with anthracycline, taxane, capecitabine, and several hormonal therapies. Lung metastasis was newly observed after sequential treatment of MBC for 6 years. Although the disease was resistant to chemotherapy of eribulin, vinorelbine, and bevacizumab with paclitaxel, it responded well to the treatment of MMC/MTX, which continued for 7 months. This case suggests that MMC/MTX could be an effective treatment for MBC patients when the disease progressively develops even after aggressive treatment with multiple regimens.