In the North Thames Mainstreaming of Breast Cancer Genetic Testing (NT-MBGT) programme, we piloted testing for breast cancer susceptibility genes (BCSGs) in unselected breast cancer (BC) patients, deploying a clinician-light 'BRCA-DIRECT' mainstreaming pathway; this included home saliva-testing and consent with postal return, written and digital materials, with full access to a Genetic Counsellor Telephone helpline. Across 14 National Health Service (NHS) breast oncology units, we successfully tested 3515 newly-diagnosed BC patients with high levels of patient and breast healthcare professional (HCP) satisfaction and genetics HCPs reporting decreases in service referrals. The pick-up rate of gPVs was 4.7% (166 germline Pathogenic Variants (gPVs) across seven BCSGs). Examining application of current NHS eligibility criteria to the unselected cohort, testing would have been offered to 20.6% of patients with identification of 49.2% of gPVs in high penetrance (HP)-BCSGs (BRCA1/BRCA2/PALB2) and 18.2% of gPVs in intermediate penetrance-BCSGs (CHEK2/ATM/RAD51C/RAD51D). Designing 'Ultra-simple' eligibility criteria suitable for mainstreaming, detection (sensitivity) could be improved to 81.1% and 70.4% respectively, whilst increasing testing to 49.7% of BC cases. Evidence from the NT-MBGT programme demonstrates that expanding BCSG-testing via a clinician-light pathway is acceptable and feasible, without increasing the burden on limited breast and genetics workforce, and has high satisfaction.