Background: Medical adherence is often higher in clinical trials than in real world practice. The aim of this study was to investigate the effects of traditional Chinese medicine (TCM) on medical adherence to hormonal therapy (HT) and survival outcome in ER (+) breast cancer patients in Taiwan. Subjects and methods: Using a nationwide longitudinal population-based database, we enrolled patients with newly diagnosed ER-positive breast cancer who had received HT, and followed for up to 5 years (N = 872). Medication adherence in terms of medication possession ratios (MPR) and patient outcome were evaluated with or without TCM exposure. We applied logistic regression and Cox proportional hazards (PH) analysis to identify factors, including TCM exposure, associated with adherence to HT and mortality. Results: MPR to HT in general decreased over the 5-year period post breast cancer diagnosis. Both TCM and MPR to HT ≥ 80% were significantly associated with reduced risk of breast cancer-associated mortality. Subgroup analysis revealed that TCM annual visits ≥ 3 times with CHP prescription 1~90 days per year affected mortality reduction most significantly (HR: 0.26; 95% CI = 0.08–0.83; p < 0.05) compared to other TCM use. In contrast, using TCM (either short-term or long-term) was not associated with MPR in HT. Conclusions: Our results supported the potential advantage of TCM on breast cancer-associated mortality, whereas TCM use does not compromise medical adherence to HT. This study offers important insights in integrative therapy for HT in patients with estrogen receptor (+) breast cancer.