Glioblastoma is the most aggressive primary brain tumor, with a median survival of 15 months with treatment. Standard-of-care (SOC) consists of resection, radio- and chemotherapy. Clinical trials involving PD-1 inhibition with nivolumab combined with SOC failed to increase survival. A quantitative understanding of the interactions between the tumor and its immune environment that drive treatment outcomes is currently lacking. As such, we developed a mathematical model of tumor growth that considers CD8+ T cells, pro- and antitumoral tumor-associated macrophages and microglia (TAMs), SOC, and nivolumab. Using our model, we studied five TAM-targeting strategies currently under investigation for solid tumors. Our results show that PD-1 inhibition fails due to a lack of CD8+ T cell recruitment during treatment, explained by TAM-driven immunosuppressive mechanisms. Our model predicts that while reducing TAM numbers does not improve prognosis, altering their functions to counter their protumoral properties has the potential to considerably reduce post-treatment tumor burden. In particular, restoring antitumoral TAM phagocytic activity through anti-CD47 treatment in combination with SOC was predicted to nearly eradicate the tumor. By studying time-varying efficacy with the same half-life as the anti-CD47 antibody Hu5F9-G4, our model predicts that repeated dosing of anti-CD47 provides sustained control of tumor growth. We propose that targeting TAMs by enhancing their antitumoral properties is a highly promising avenue to treat glioblastoma and warrants future clinical development. Together, our results provide proof-of-concept that mechanistic mathematical modeling can uncover the mechanisms driving treatment outcomes and explore the potential of novel treatment strategies for hard-to-treat tumors like glioblastoma.