CD8 T cell responses against different tumor neoantigens occur simultaneously, yet little is known about the interplay between responses and its impact on T cell function and tumor control. In mouse lung adenocarcinoma, we found that immunodominance is established in tumors, wherein CD8 T cell expansion is predominantly driven by the antigen that most stably binds MHC. T cells responding to subdominant antigens were enriched for a TCF1⁺ progenitor phenotype correlated with response to immune checkpoint blockade (ICB) therapy. However, the subdominant T cell response did not preferentially benefit from ICB due to a dysfunctional subset of TCF1⁺ cells marked by CCR6 and Tc17 differentiation. Analysis of human samples and sequencing datasets revealed that CCR6⁺ TCF1⁺ cells exist across human cancers and are not correlated with ICB response. Vaccination eliminated CCR6⁺ TCF1⁺ cells and dramatically improved the subdominant response, highlighting a strategy to optimally engage concurrent neoantigen responses against tumors.