Lymphocyte-activation gene 3 (LAG3) is an immune checkpoint receptor and inhibitory regulator of T-cells. Here, we analyzed the prevalence of LAG3 rs870849 in B-cell lymphoma patients and the treatment outcomes according to the LAG3 genetic background and discovered that LAG3 germline variants may affect the risk of developing lymphoma and also affect the treatment outcome of DLBCL patients in the current CD19 CAR-T cell therapies. The LAG3 rs870849 was prevalent at high frequency in DLBCL patients. Significant differences in treatment outcomes to CAR-T cell therapy emerged in LAG3 I455hom versus I455Thet and T455hom carriers. The overall and complete response rates to CAR-T cell therapy were lower in the I455hom genetic subgroup with median PFS in the I455hom of 2 versus 20 months in the T455hom and I455Thet subgroups (p = 0.025). Median OS was 6 months in the LAG3 I455hom versus 41 months in the T455hom and I455Thet subgroups (p = 0.007). LAG3 rs870849 may affect treatment outcome in CAR-T cell therapy, with favorable outcomes in T455 carriers. Specific combinations of CTLA4 and LAG3 germline variants may cooperate to affect the response to CAR-T cell therapy.