Background: Guideline-concordant care (GCC) of breast cancer greatly improves survival. Women with mental illness experience worse survival after breast cancer; in this study, we examined whether women with mental illnesses pre-existing their breast cancer diagnosis receive GCC for breast cancer as often as women without. Methods: We used Surveillance and Epidemiology and End Results (SEER) cancer registry and Medicare claims (SEER-Medicare) to select cases of women (67+ years old) with Stage I-III breast cancers (n = 89,172). Mental illness was measured through diagnostic codes within 2 years before cancer diagnosis and categorized as serious mental illness (SMI: schizophrenia, bipolar disorder, depression with psychosis, and other psychotic disorders); depression or anxiety; or other mental illnesses. To determine receipt of GCC we used the National Comprehensive Cancer Network’s (NCCN) treatment guidelines, commonly referenced by oncologists as best practices. Outcomes included 1. surgery and radiation completion for all cancers (complete/incomplete treatment/no surgery); 2. surgery, radiation completion, and chemotherapy initiation (complete/incomplete/no surgery) for triple negative and HER2+ breast cancers; and 3. radiation completion after mastectomy for Stage III cancers with lymph involvement. We used generalized ordinal logistic regression to compare outcomes with mental illness categories, controlling for demographic, cancer-related, and clinical factors. Results: We found that 28.8% of women in this study had at least one diagnosis of a mental illness in the two years prior to their breast cancer diagnosis and 1.7% had SMI. Women with SMI are more likely to not receive surgery than women without (OR = 1.24, CI = 1.02-1.60). Women with mental illnesses have a higher risk of not completing radiation after breast conserving surgery (SMI: OR = 1.24, CI = 1.01-1.30, Depression and anxiety: OR = 1.11, CI = 1.06-1.16, other mental illnesses: OR = 1.09 CI = 1.01-1.16). Women with SMI and triple negative or HER2+ cancers are more likely to not complete all treatment (OR = 1.65, CI = 1.22-2.24). Conclusions: Women with mental illnesses may be at higher risk for incomplete treatment or lack of treatment initiation, especially for multi-part treatment, such as completion of radiation and initiation of chemotherapy, which may contribute to worse survival outcomes. Breast cancer and mental illness are both common illnesses among older women in the United States. Health systems should consider strategies for improving GCC among women with mental illness and breast cancer.