BACKGROUND: Omission of sentinel lymph node biopsy (SLNB) in older women with clinically node-negative, hormone receptor-positive (HR+) early-stage breast cancer undergoing lumpectomy is accepted, given established low rates of regional recurrence. The safety of omitting SLNB in women undergoing mastectomy is unknown and may differ depending on extent of breast disease and variation in radiotherapy use. PATIENTS AND METHODS: From 2006 to 2018, 123 cTis and 328 cT1-2 HR+/HER2- tumors from 410 women aged ≥ 70 years who underwent mastectomy and SLNB were included (41 bilateral cases). The rate of nodal positivity and effect of nodal positivity on adjuvant therapy use were examined. RESULTS: Median age was 74 years; 21% of patients had positive sentinel lymph nodes, 7% had micrometastases, and 14% had macrometastases. Of cases of cTis tumors, 31% were upstaged to invasive carcinoma; 1% had macrometastases. Fewer cases of cT1 than cT2 tumors had macrometastases [13% (26/200) versus 29% (37/128); p < 0.001]. Eight percent of patients with pT1 tumors (18/228) and 27% of patients with pT2 tumors (30/113) received chemotherapy. Most patients with pT1, pN1 disease (78%; 25/32) did not receive chemotherapy. Rates of locoregional recurrence were similar between patients with cT1 or cT2 tumors with and without nodal metastases (median follow-up, 4.5 years). CONCLUSIONS: Women aged ≥ 70 years with cTis and cT1N0 HR+/HER2- tumors who underwent mastectomy had low rates of nodal positivity, similar to rates reported for lumpectomy. Given this and the RxPONDER results, omission of SLNB may be considered, as findings are unlikely to alter adjuvant therapy recommendations.