We found that aging mouse mammary stem cells (MaSCs) form mammary ducts with higher frequency of early neoplastic lesions than those formed by young MaSCs. Aging mouse mammary glands also show increased gene signatures of inflammation and immune responses, and enriched MTORC1 signalling activity in our investigation involving various molecular, cellular, in vivo, and bioinformatics approaches including single cell transcriptomic studies. Treatment of mouse models of mammary tumors with an MTORC1 inhibitor everolimus significantly reduced tumor incidence and the frequency of invasive mammary tumors. We recently investigated whether targeting mTORC1 pathway is effective in abrogating mammary stem cell (MaSC) function and early-stage invasive breast cancer progression markers. A short-term treatment of women with Stage 0 breast tumors with a low dose of sirolimus in a phase II clinical study significantly abrogated mammary stem cell activity, particularly in postmenopausal patients. It did not affect the frequency of luminal progenitors but decreased their self-renewal capacity. The treatment also significantly diminished prognostic biomarkers associated with mammary gland inflammation and breast cancer progression from ductal carcinoma in situ to invasive breast cancer. Overall, these findings indicate a link from MTOR signalling to mammary stem and progenitor cell activity and cancer progression. MTOR inhibitors may have potential for prevention of invasive breast cancer.