Objective: Although Socioeconomic status (SES), race/ethnicity, surgical type, and treatment delays are associated with breast cancer mortality outcomes, studies on these associations have been contrasting. This study examined the racial/ethnic and SES differences in surgical treatment types and treatment delays. Also, we quantified the extent to which these differences explained the racial/ethnic disparities in breast cancer mortality., Methods: We studied 290,066 women 40 + years old diagnosed with breast cancer between 2010 and 2017 identified from the Surveillance, Epidemiology, and End Results database. We performed logistic regression models to examine the association of SES and race/ethnicity with surgical treatment type and treatment delays. We performed mediation analysis models to quantify the extent to which mortality differences were mediated by treatment, sociodemographic, and clinicopathologic factors., Results: Non-Hispanic (NH) Black [Odds ratio (OR) = 1.16, 95% CI 1.13-1.19] and Hispanic women [OR = 1.27, 95% CI 1.24-1.31] were significantly more likely to undergo mastectomy compared to NH White women. Similarly, NH Black and Hispanic women had higher odds of delayed treatment than NH Whites. Patients in the highest SES quintile, compared to those in lowest the lowest, were less likely to experience breast cancer-specific mortality (BCSM). Variations in treatment, SES, and clinicopathological factors significantly explained 70% of the excess BCSM among NH Blacks compared to their NH White counterparts., Conclusions: Bridging the gap of access to adequate healthcare services for all to diminish the disproportionate burden of breast cancer would require a multifactorial approach that addresses several biological and social factors that cause these differences., (© 2023. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)