Background: During the COVID-19 pandemic, health resources were stretched, access was impacted by lockdowns and there were concerns about exposure to the virus during visits to hospitals. The purpose of this study was to examine how breast cancer treatments (presentation, surgery, radiotherapy, chemotherapy and/or endocrine therapy) changed or were adapted during the early phase of the pandemic. Methods: A systematic review was conducted using PRISMA guidance. Eligible studies presented original data reporting changes to early breast cancer treatment by comparing ‘pandemic’ treatment to a ‘pre-pandemic’ cohort or to ‘ideal’ treatment of individual cases. Data were extracted into evidence tables and narrative synthesis was used to analyse results.   Results: Fifteen studies with paired design were eligible. These reported outcomes for 6,353 people treated for early breast cancer (January 2020–June 2021). All studies reported some change to treatment due to the pandemic. The nature of reported changes was inconsistent. Changes included: tumours more advanced at presentation compared to pre-pandemic, increase in breast conserving surgery;  increase in simple mastectomy (without breast reconstruction); trend towards increased wait times, delays to start of treatment, shorter post-operative hospital stay and hypofractionation or omission of radiotherapy. Centres used more or less neoadjuvant chemotherapy or endocrine therapy. Conclusion: In the early stage of the pandemic, fewer early-stage breast cancer cases were treated many centres. Treatment for breast cancer was impacted and various local solutions were developed. These included less complicated breast surgery, increased use of neoadjuvant therapy, and changes to radiotherapy regimens. Surgery was frequently delayed and breast reconstruction was often unavailable. These results have implications for breast cancer services during the pandemic recovery as a ‘catch-up’ increase in cancer diagnoses is expected. Women may wish to access breast reconstruction, unavailable due to COVID-19. The impact of changes to treatment on long-term quality of life should be evaluated.