The biological and functional heterogeneity between tumors—both across and within cancer types—poses a challenge for immunotherapy. To understand the factors underlying tumor immune heterogeneity and immunotherapy sensitivity, we established a library of congenic tumor cell clones from an autochthonous mouse model of pancreatic adenocarcinoma. These clones generated tumors that recapitulated T cell-inflamed and non-T-cell-inflamed tumor microenvironments upon implantation in immunocompetent mice, with distinct patterns of infiltration by immune cell subsets. Co-injecting tumor cell clones revealed the non-T-cell-inflamed phenotype is dominant and that both quantitative and qualitative features of intratumoral CD8+ T cells determine response to therapy. Transcriptomic and epigenetic analyses revealed tumor-cell-intrinsic production of the chemokine CXCL1 as a determinant of the non-T-cell-inflamed microenvironment, and ablation of CXCL1 promoted T cell infiltration and sensitivity to a combination immunotherapy regimen. Thus, tumor cell-intrinsic factors shape the tumor immune microenvironment and influence the outcome of immunotherapy. Display omitted •Generated a library of congenic pancreatic cancer cell clones derived from KPC mice•Each tumor elicited unique immune infiltration correlating with therapeutic response•Tumors lacking T cells exhibit different epigenetic and transcriptomic status•CXCL1 was increased in therapy-resistant tumors that lacked T cell infiltration Using a library of pancreatic cancer cell clones, Li et al. identify heterogeneous and multifactorial pathways regulating tumor-cell-intrinsic mechanisms that dictate the immune microenvironment and thereby responses to immunotherapy. This tumor clone library provides a tool for identifying new targets responsible for thwarting responses to immunotherapy in resistant tumors.