Women with BRCA1 and BRCA2 mutations have an elevated risk of breast cancer and ovarian cancer, but also of developing second primary breast cancer. BRCA1/2 mutation carriers with breast cancer must choose between breast conservation (BCT) and mastectomy (M) yet data on outcomes are limited. The purpose of this study is to compare long-term outcome following BCT and M in BRCA1/2 carriers. 655 women with BRCA1/2 mutations diagnosed with breast cancer and treated with BCT (n = 302) or M (n = 353) were identified and underwent follow-up to assess local, regional, and systemic recurrence. Local failure as first failure was significantly more likely in those treated with BCT compared to M, with a cumulative estimated risk of 23.5 vs. 5.5%, respectively, at 15 years (P < 0.0001); 15-year estimates in carriers treated with BCT and chemotherapy was 11.9% (P = 0.08 when compared to M). Most events appeared to be second primary cancers rather than failure to control the primary tumor. The risk of contralateral breast cancer was high in all groups, exceeding 40%, but was not statistically significantly different by use of adjuvant radiotherapy (RT) or not, suggesting no added risk from scatter RT at 10 and 15 years. There were no differences seen in regional or systemic recurrences between the BCT and M groups, and no difference in overall survival. In conclusion, BRCA1/2 mutation carriers with breast cancer have similar survival whether treated with M or BCT. However, women undergoing BCT have an elevated risk of a second in-breast event that is significantly reduced in the presence of chemotherapy. Contralateral breast cancer events are very common.