Background: Recent studies have demonstrated a decline in cancer screening and diagnosis during the COVID-19 pandemic. This study explored trends in the diagnosis and management of eBC at a sample of cancer clinics across the US early on in the pandemic. Methods: Patients were selected from the Flatiron Health Research Database (FHRD), an electronic health record-derived de-identified database comprising approximately 280 US cancer clinics (̃800 sites of care). Eligible patients had an ICD code for breast cancer, at least two clinical encounters, and a confirmed eBC (Stage I-III) diagnosis from unstructured documents. Patients were selected into two cohorts based on diagnosis date: a) COVID-19 era cohort diagnosed between February 1, 2020 through June 30, 2020 and b) pre-COVID-19 era cohort diagnosed from February 1, 2019 through June 30, 2019. Descriptive statistics were used to assess diagnosis trends in each time frame. Initial treatment received following eBC diagnosis was categorized as surgery, radiation or systemic therapy and was compared between the two cohorts. Initial treatment modalities for each cohort were further stratified by clinical stage and biomarker subtype (HER2+, HR+/HER2-, triple negative [TN] or unknown). Results: A total of 278 and 253 patients were selected for the pre-COVID-19 era and COVID-19 era cohorts, with a median age at diagnosis of 65 and 64 years, respectively. A 35% decrease in the number of eBC diagnoses was observed in April/May 2020 compared to March 2020, yet this reduction in diagnoses was not observed during the equivalent months in the pre-COVID-19 era cohort. Compared to the pre-COVID-19 era, a greater proportion of patients diagnosed with eBC during the COVID-19 era initiated systemic therapy as their first treatment modality (16.5% vs 29.6%) including patients with HER2+ (27.5% vs. 60%), HR+/HER2- (13.5% vs. 24.9%) and TN (30.8% vs. 40.0%) disease. This trend was observed in patients with stage I (11.7% vs. 24.1%) or II (55.9% vs. 73.0%) but not in patients with stage III (81.2% vs. 77.3%) eBC. Notably, among patients with HR+/HER2- eBC who received systemic therapy as their first treatment, endocrine therapy was most commonly used in keeping with recent recommendations from professional societies due to COVID-related anticipated surgical delays. Conclusions: This study demonstrates that COVID-19 was associated with a decreased incidence of eBC which could be, at least in part, attributed to previously reported delays in routine screening and pandemic healthcare utilization. Further efforts are required to understand who was affected by these delays and the impact on cancer outcomes. Follow-up data are needed to understand if the observed trends in cancer screening and treatment persist and their impact on long-term cancer outcomes.