Background: Results of the American College of Surgeons Oncology Group (ACOSOG) Z0011 trial supports omission of completion axillary lymph node dissection (CLND) after breast-conservation surgery with a positive sentinel lymph node biopsy (SLNB). We hypothesized that CLND also does not impact outcomes in women with clinically node-negative (cN0), pathologically node-positive breast cancer undergoing mastectomy. Materials and Methods: A single-institution retrospective review was performed of patients with SLN-positive breast cancer treated from July 1999 through May 2018. Clinicopathologic and outcome data were collected. Patients with SLNBs were compared with those receiving SLNB and CLND. The Kruskal-Wallis, chi-square, and Fisher exact tests were used to assess for differences between continuous and categorical variables. The log-rank test was used for time-to-event analyses, and Cox proportional hazards models were fit for locoregional and distant recurrence and overall survival (OS). Results: Of 329 patients with SLN-positive breast cancer undergoing mastectomy, 60% had CLND (n=201). Median age at diagnosis was 53 years (interquartile range [IQR], 46–62 years). The median number of SLNs sampled was 3 (IQR, 2–4), and the median number of positive SLNs was 1 (IQR, 1–2). Patients receiving CLND had higher tumor grades (P=.02) and a higher proportion of hormone receptor negativity (estrogen receptor, 19%; progesterone receptor, 27%; both P=.007). A total of 44 patients (22%) had increased N stage after CLND. Median follow-up was 51 months (IQR, 29–83 months). No association was found between CLND and change in OS and locoregional or distant recurrence. Completion of postmastectomy radiotherapy was associated with improved OS (P=.04). Conclusions: CLND is not significantly correlated with reduced recurrence or improved OS among patients who have cN0, SLN-positive breast cancer treated with mastectomy. CLND was significantly correlated with receipt of adjuvant systemic therapy. Completion of postmastectomy radiotherapy was associated with improved OS.