IMPORTANCE: The 2009 US Preventive Services Task Force breast cancer screening guideline changes led to decreases in screening mammography, raising concern about potential increases in late-stage disease and more invasive surgical treatments. OBJECTIVE: To investigate the incidence of breast cancer by stage at diagnosis and surgical treatment before and after the 2009 guideline changes. DESIGN, SETTING, AND PARTICIPANTS: This population-based, epidemiologic cohort study of women aged 40 years or older used 2004 to 2019 data from the National Cancer Institute's Surveillance, Epidemiology, and End Results Program. Age- and stage-specific breast cancer incidence rates and the proportion of breast cancers treated by partial mastectomy, total mastectomy, and total mastectomy with reconstruction were calculated. Data analyses were conducted from August 2023 to February 2024. INTERVENTIONS OR EXPOSURES: Age group (40-49, 50-74, and ≥75 years). MAIN OUTCOMES AND MEASURES: Annual percent changes (APCs) in stage-specific breast cancer incidence and proportions of cases treated with each surgery type. RESULTS: This cohort study included 2 022 250 women (354 263 [17.5%] aged 40-49 years, 1 279 542 [63.2%] aged 50-74 years, and 388 445 [19.2%] aged ≥75 age group, from a total of 2 023 541 women) diagnosed with breast cancer. Rates of in situ breast cancer decreased since 2009 (eg, APC, -0.69 [95% CI, -2.77 to -0.18] for women aged 50-74 years). Localized breast cancer rates increased steadily during 2004 to 2019 in women aged 40 to 74 years (eg, APC, 1.18 [95% CI, 1.02-1.34] for women aged 50-74 years), with no evidence of a change in trend during the study period. Regional cancer rates decreased or did not change. Distant cancer rates were flat since 2012 among women aged 40 to 74 years and increased steadily for those 75 years or older during 2004 to 2019 (APC, 1.40 [95% CI, 1.00-1.82]). The proportion of cases treated with partial mastectomy decreased during 2004 to 2012 (eg, APC, -0.77 [95% CI, -2.96 to -0.03] among women aged 50-74 years with localized cancer), whereas the proportion of cases treated with total mastectomy with reconstruction increased (eg, APC, 20.17 [95% CI, 16.50-33.16]). During 2012 to 2019, the proportion of cases treated with total mastectomy decreased (eg, APC, -2.44 [95% CI, -3.45 to -1.61] for women aged 50-74 years with localized cancer), and the proportion of cases treated with partial mastectomy increased (eg, APC, 1.70 [95% CI, 0.90-4.08] for women aged 50-74 years). CONCLUSIONS AND RELEVANCE: In this cohort study, in situ breast cancer decreased since 2009, consistent with decreasing use of screening mammography since the 2009 guideline changes, but this decrease did not appear to have translated to more advanced breast cancer stages at diagnosis or decreases in the proportion of cases treated with partial mastectomy. Further research is needed to understand the long-standing increase in localized invasive breast cancer and the decrease in regional invasive breast cancer observed during the past 20 years in the context of decreased breast cancer screening.