Rational combinatorial therapeutic strategies have proven beneficial for the management of cancer. Recent success of checkpoint blockade in highly immunogenic tumors has renewed interest in immunotherapy. Regulatory T (T reg) cells densely populate solid tumors, which may promote progression through suppressing anti-tumor immune responses. We investigated the role of T reg cells in murine mammary carcinogenesis using an orthotopic, polyoma middle-T antigen-driven model in Foxp3^DTR knockin mice. T reg cell ablation resulted in significant determent of primary and metastatic tumor progression. Importantly, short-term ablation of T reg cells in advanced spontaneous tumors led to extensive apoptotic tumor cell death. This anti-tumor activity was dependent on IFN-γ and CD4⁺ T cells but not on NK or CD8⁺ T cells. Combination of T reg cell ablation with CTLA-4 or PD-1/PD-L1 blockade did not affect tumor growth or improve the therapeutic effect attained by T reg cell ablation alone. However, T reg cell targeting jointly with tumor irradiation significantly reduced tumor burden and improved overall survival. Together, our results demonstrate a major tumor-promoting role of T reg cells in an autochthonous model of tumorigenesis, and they reveal the potential therapeutic value of combining transient T reg cell ablation with radiotherapy for the management of poorly immunogenic, aggressive malignancies.