Objective: Many differences between U.S. military beneficiaries and the U.S. general population, including differences in health care access, are known factors affecting invasive breast cancer outcomes. Thus, comparing the two populations for any outcome differences and their contributing factors may provide insights to breast cancer prognosis. Methods: Using a marginal Cox proportional hazards regression model, we compared diseasespecific survival (DSS) and 5-year DSS rates between 418 patients from the Clinical Breast Care Project at the Walter Reed National Military Medical Center (CBCP-WR) and a set of 1:5 randomly matched patients from the Surveillance, Epidemiology, and End Results program. Patients were compared in the “demographic model” (adjusted by diagnosis year, age, and race) and the “overall model” (further adjusted by estrogen receptor, progesterone receptor, stage, and grade). Results: In the “overall model,” CBCP-WR patients were less likely overall to die from breast cancer (hazard ratio [HR] = 0.631, 95% confidence interval [CI] = 0.437–0.911; p = 0.014). This increase in survival was also significant in African American patients (HR = 0.524, 95% CI = 0.277–0.992; p = 0.047) and patients older than 50 (HR = 0.511, 95% CI = 0.306–0.854; p = 0.010). The advantage in 5-year DSS rate for CBCP-WR patients was 5.3% (93.1% vs. 87.8%; p < 0.001) in the “demographic model” and 3.4% (91.3% vs. 87.9%; p = 0.018) in the “overall model.” Conclusion: CBCP-WR patients demonstrated significantly better DSS over matched SEER patients. Although a portion of the outcome disparity, i.e., 36% of the 5.3% DSS rate difference, could be explained by differences in tumor characteristics, the cause(s) behind the majority of the disparity has yet to be identified. Identification and further analysis of contributing factors to survival differences have the potential to improve clinical practice and outcomes for invasive breast cancer patients.