Objective To identify key organizational approaches associated with underuse of breast cancer care. Setting Nine New York City area safety‐net hospitals. Study Design Mixed qualitative–quantitative, cross‐sectional cohort. Methods We used qualitative comparative analysis (QCA) of key stakeholder interviews, defined organizational “conditions,” calibrated conditions, and identified solution pathways. We defined underuse as no radiation after lumpectomy in women <75 years or mastectomy in women with ≥4 positive nodes, or no systemic therapy in women with tumors ≥1 cm. We used hierarchical models to assess organizational and patient factors’ impact on underuse. Principal Findings Underuse varied by hospital (8–29 percent). QCA found lower underuse sites designated individuals to track and follow‐up no‐shows; shared clinical information during handoffs; had fully integrated electronic medical records enabling transfer of responsibility across specialties; had strong system support; allocated resources to cancer clinics; had a patient‐centered culture paying close organizational attention to clinic patients. High underuse sites lacked these characteristics. Multivariate modeling found that hospitals with strong approaches to follow‐up had low underuse rates (RR = 0.28; 0.08–0.95); individual patient characteristics were not significant. Conclusions At safety‐net hospitals, underuse of needed cancer therapies is associated with organizational approaches to track and follow‐up treatment. Findings provide varying approaches to safety nets to improve cancer care delivery.