Background: Breast cancer (BC) is a malignancy with a wide array of histopathological characteristics and tumor heterogeneity leading to major clinical implications for management. With the recent approval of antibody-drug conjugates (ADCs) for human epidermal growth factor receptor 2 (HER2)-low advanced BC, there is increased significance for characterizing the spectrum of HER2 expression in the curative setting. Methods: We conducted a single institution retrospective review of patients with BC who received neoadjuvant systemic therapy (NAST) followed by definitive surgical resection from 2008 to 2022. The initial tumor biopsies were tested for estrogen receptor (ER), progesterone receptor (PR), and HER2 status per ASCO/CAP guidelines and repeated on the residual tumor tissue after NAST. HER2 status was categorized into HER2-negative (IHC 0), HER2-low (IHC 1 or IHC 2/ Fluorescence in situ hybridization [FISH] negative), and HER2-positive (IHC 2/ FISH positive or IHC 3). Concordance and discordance with HER2 status in the pre and post NAST tumor samples were analyzed using descriptive statistics. Results: Of 340 patients with BC who received NAST, 55 patients with residual disease had pre and post treatment biomarker data available for analysis. Prevalence of HER2-low was 70.9 % (39/55) in the pretreatment tumors while it was 58.2% (32/55) in the post treatment tumors. 40% of tumors (22/55) had HER2 discordance while 60% (33/55) demonstrated concordance with HER2 expression after NAST. Among the changes in HER2 status, a shift from HER2-low to HER2-negative status occurred most frequently in 18.2% of tumors (10/55). In tumors with HER2 discordance, 36.4% (8/22) were also noted to have changes in ER or PR status while this was seen in 48.5% (16/33) of tumors with HER2 concordance. For rates of recurrence, 22.7% (5/22) of patients in the HER2 discordant group had relapsed compared to 18.2% (6/33) patients in the HER2 concordant group. Conclusions: Dynamics of HER2 expression is increasingly important in the rapidly changing therapeutic landscape of BC. Our study shows high prevalence of HER2-low expression in tumors with residual disease after NAST and frequent conversion of HER2-low to HER2-negative status with NAST. Further studies are warranted to determine the clinical and therapeutic relevance of biomarker evolution and tumor heterogeneity in the curative setting.