Background: Understanding the patterns of healthcare utilization and costs during the initial phase of care (12 months after breast cancer [BC] diagnosis) in older women (aged ≥65 years) is crucial in the allocation of Medicare resources. The objective of this study was to determine healthcare utilization and costs during the initial phase of care in older, female, Medicare fee-for-service beneficiaries diagnosed with BC, and to determine the factors associated with higher costs. Methods: A retrospective observational study using the SEER-Medicare linked database was conducted in 69,307 women aged ≥66 years diagnosed with primary incident BC in 2003–2009 to determine healthcare utilization, average costs, and costs for specific services during the initial phase of care. Generalized linear model regression was conducted to identify the factors associated with higher costs in a multivariate framework. Results: A total of 96% of women were treated with surgery during the initial phase of BC care, whereas 21% and 54% underwent chemotherapy and radiotherapy, respectively. Costs during the initial phase of care totalled $28,075 in 2012 USD, comprising $13,344 for physician services and $7,456 for outpatient services. Factors associated with higher costs during the initial phase of care were younger age (66–69 years), African American race, higher household income, advanced stages of BC, initial BC treatment, higher number of primary care physician visits, and presence of comorbidities and/or a mental condition. Conclusions: The economic burden of BC is substantial during the initial phase of care. Physician and outpatient services accounted for the highest proportion of costs. Predisposing factors, need-related factors, healthcare use, and external environmental healthcare factors significantly predicted costs during the initial phase of care.