We performed a registry study on therapy-related myeloid neoplasm (t-MN), both therapy-related myelodysplastic syndrome (t-MDS) and acute myeloid leukemia (t-AML) following treatment for breast cancer who underwent a first allogeneic hematopoietic cell transplant (allo-HCT). Of 252 identified female patients (median age 57 years), 77% were transplanted for t-AML and 23% for t-MDS, with a median time from breast cancer diagnosis to the diagnosis of tMN and subsequent allo-HCT of 3.7 and 4.6 years, respectively. At transplant, 191 patients were in remission for breast cancer, while 4 were not (57 missing). T-MN was in a complete remission at the time of transplant in 67% of patients. 2-year overall survival, relapse free-survival, relapse incidence and non-relapse mortality were 50%, 45%, 33%, and 22%, respectively. Multivariable analysis revealed that if the t-MN was not in CR pre-transplant, this was associated with lower OS, RFS, and a higher relapse incidence. Seventeen cases of breast cancer recurrence were recorded after a median of 2.4 years post-transplant, and relapse of primary breast cancer accounted for 7% of deaths. This study indicates that allo-HCT for t-MN following treatment for breast cancer shows encouraging transplant outcomes. The incidence of breast cancer relapse post-transplant remains a cause for concern.