Background: Management of axillary lymph nodes in breast cancer has undergone significant change over the past decade through landmark clinical trials. This study aimed to assess national practice patterns in axillary management in patients undergoing upfront mastectomy and examines what guides provider recommendations. Methods: A national case-based survey study was performed of surgeons and radiation oncologists from July to August 2020. Surgeons were identified through the American Society of Breast Surgeons (ASBrS) after review and approval by the ASBrS Research Committee, and radiation oncologists were identified through an institutional database. Both descriptive and comparative statistical analyses were performed. Results: Overall, 994 providers responded—680 surgeons and 314 radiation oncologists. Surgeons were older and in practice longer (p < 0.05) and treated a higher percentage of breast patients (81% vs. 40%, p < 0.001). Most surgeons were hospital-employed (43%), whereas most radiation oncologists were in private practice (40%; p < 0.001). Fifty-two percent of surgeons routinely send sentinel lymph nodes (SLNs) for frozen section (52%) during mastectomy, of which 78% proceed directly to axillary lymph node dissection (ALND) if positive. There was significant variability in treatment recommendations between the two groups among the hypothetical cases (p < 0.001). In the setting of low disease burden in the SLNs, > 30% of surgeons recommended ALND, while radiation oncologists recommend axillary radiotherapy over axillary clearance (p < 0.001). Conclusion: There is significant heterogeneity in the management of the axilla in mastectomy patients with pathologically positive SLNs, both between and among surgeons and radiation oncologists. Efforts should be made to assist both groups in identifying de-escalation opportunities to ensure that mastectomy patients with positive SLNs are treated appropriately.