Chimeric antigen receptor T cell (CAR-T) therapy directed at CD19 produces durable remissions in the treatment of relapsed/refractory non-Hodgkin's lymphoma (NHL). Nonetheless, many patients receiving CD19 CAR-T cells fail to respond for unknown reasons. To reveal changes in 4-1BB-based CD19 CAR-T cells and identify biomarkers of response, we employed single cell RNA sequencing and protein surface marker profiling of patient CAR-T cells pre- and post-infusion into NHL patients. At the transcriptional and protein levels, we note the evolution of CAR-T cells toward a non-proliferative, highly differentiated, and exhausted state, with an enriched exhaustion profile in CAR-T cells of patients with poor response marked by TIGIT expression. Utilizing in vitro and in vivo studies, we demonstrate TIGIT blockade alone improves the anti-tumor function of CAR-T cells. Altogether, we provide evidence of CAR-T cell dysfunction marked by TIGIT expression driving a poor response in NHL patients.