Background: Estrogen receptor (ER)+HER2- breast cancer has a good prognosis and is known to have more prolonged survival than other subtypes even after developing distant metastasis (DM). Predicting and improving long-term survival after DM is important, especially for young breast cancer patients with a long-life expectancy. This study aimed to analyze factors associated with long-term survival after DM in young ER+HER2- breast cancer (age ≤ 45). Methods: We retrospectively collected 2,951 young ER+HER2- breast cancer patients (age ≤ 45) who received primary surgery at three academic institutes between January 2000 and December 2011. Patients with survival data for at least 60 months after developing DM were analyzed. The patients must have received endocrine therapy for more than 24 months or until DM. Patients who had received neoadjuvant chemotherapy were excluded. Cox proportional hazard and binary logistic models were used to identify the factors related to post-metastatic overall survival (PMOS) and those associated with short- ( < 60 months) versus long- (≥ 60 months) term survival after DM. Results: Of the 245 patients with DM, 188 had expired. The median follow-up was 117 (12–271) months from primary surgery and 38 (0–196) months from DM. In multivariate Cox analysis, lymph node (LN) metastasis of the primary tumor was identified as a risk factor of PMOS (hazard ratio [HR], 1.84; 95% confidence intervals [CI], 1.28–2.64; P = 0.001). LN metastasis of the primary tumor was a risk factor for short-term ( < 60 months) survival (odds ratio [OR], 3.36; 95% CI, 1.66–6.79; P < 0.001), and histologic grade (HG) I of the primary tumor was a favorable factor for long-term (≥ 60 months) survival (OR, 0.15; 95% CI, 0.03–0.76; P = 0.026) on multivariate analysis. In addition, early recurrence ( < 60 months) (OR, 1.90; 95% CI, 1.04–3.47; P = 0.037), accompanying locoregional recurrence (LRR) (OR, 4.56; 95% CI, 1.96–10.59; P < 0.001), and metastasis at multiple sites (OR, 2.90; 95% CI, 1.56–5.42); P < 0.001) were negative indicators for long-term survival after DM. Conclusions: This study revealed that primary tumor characteristics such as LN metastasis and HG are sustained as prognostic factors for long-term survival after developing DM in young ER+HER2- breast cancer. Early recurrence ( < 60 months), accompanying LRR, and metastasis at multiple sites were also poor prognostic factors.