Background: MAP3K1 (Mitogen-Activated Protein Kinase Kinase Kinase 1) participates in the MAPK signal transduction pathway, responding to a number of mitogenic and metabolic stimuli, which may be related to breast cancer susceptibility and progression. In this study, we assessed whether overexpression of MAP3K1 promotes proliferation, migration, and invasion of breast cancer cells, and thus, affects the prognosis of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative early-stage breast cancer. Methods: Two HR-positive, HER2-negative breast cancer cell lines (MCF7 and T-47D), both overexpressing MAP3K1, were transfected with MAP3K1 short hairpin RNA plasmids (shMAP3K1) and their proliferation, migration, and invasion were examined. We assessed whether shMAP3K1 affects the cell cycle and levels of downstream signaling molecules, such as ERK and NF-κB, as well as sensitizes cells in both cell lines to chemotherapeutic and hormonal agents. A total of 161 patients with HR-positive, HER2-negative T1-T2 breast cancer and 0 to 3 nodal metastases were included. The expression of MAP3K1 and phospho (p)-ERK proteins was assessed by immunohistochemistry. Results: In both cell lines (MCF7 and T-47D), shMAP3K1 significantly reduced cell growth, migration, and invasion by downregulating MMP-9 and blocking the G2/M phase of the cell cycle and its regulatory molecule cyclin B1. Besides, shMAP3K1 downregulated ERK- and NF-κB-dependent gene transcription, and enhanced sensitivities of both cell lines to doxorubicin, docetaxel, and tamoxifen. Patients with MAP3K1 overexpression exhibited significantly poor 9-year disease-free survival (DFS; 72.6% vs. 88.5%, p = 0.022) and overall survival (OS; 83.8% vs. 96.2%, p = 0.012) relative to those without MAP3K1 overexpression. Furthermore, the p-ERK expression was significantly associated with MAP3K1 expression (p < 0.001) and correlated with a poor 9-year DFS (p = 0.033) and OS (p = 0.023). Conclusions: Our results indicate that overexpression of MAP3K1 plays a major role in poor prognosis of HR-positive, HER2-negative early-stage breast cancer.