The objectives of this study are to assess the impact of pre-existing diabetes and diabetes treatment on breast cancer prognosis. 8,108 women with centrally confirmed invasive breast cancer in the Women's Health Initiative diagnosed between 1998 and 2013 were followed through the date of death or September 20, 2013. Information on diabetes and diabetes therapy were obtained via self-report and face-to-face review of current medication containers, respectively. Cox proportional hazard regression was used to estimate adjusted relative hazard ratios for overall mortality. The proportional subdistribution hazard model was used to estimate hazard ratios for breast cancer-specific mortality. Compared with women without diabetes, women with diabetes had significantly increased risk of overall mortality (HR 1.26 95 % CI 1.06-1.48), especially among those who took insulin or had longer duration of diabetes. However, diabetes was not associated with increased risk of breast cancer-specific mortality, regardless of type of treatment and duration of diabetes, despite the significant association of diabetes with unfavorable tumor characteristics. Our large prospective cohort study provides additional evidence that pre-existing diabetes increases risk of total mortality among women with breast cancer. The increased total mortality associated with diabetes was mainly driven by increased risk of dying from diseases other than breast cancer. Thus, the continuum of care for breast cancer patients with diabetes should include careful attention to CVD risk factors and other non-cancer conditions.