For interventions aimed at redressing health disparities in breast cancer to be effective, a clear understanding of the nature and causes of these disparities is required. Our questions were: what is the current evidence for ethnoracial disparities in time-to-treatment initiation and survival in breast cancer, and how are the causal mechanisms of these disparities conceptualized in the literature? A comprehensive systematic search of studies on cohorts of female patients with breast cancer diagnosed with stage I-III was performed. Directed acyclic graphs were used to describe implicit causal relationships between racial/ethnic group membership and time-to-treatment initiation and survival outcomes. This review revealed strong evidence for ethnoracial disparities in both time to treatment and survival among patients with breast cancer. Unmeasured factors identified by the authors highlighted gaps in data sources and opportunities for causal reasoning. Although the existing literature describes ethnoracial disparities, there is very limited discussion of causal mechanisms and no discussion of system-level rather than individual-level effects. Addressing established ethnoracial disparities in breast cancer requires new research that explicitly considers the causal mechanisms of potential interventions, incorporating unmeasured factors contributing to these disparities. Trial registration: PROSPERO identifier: CRD42023391901.