Phospholipase C gamma 1 (PLCG1) has been identified as the most frequently mutated gene in adult T-cell leukemia/lymphoma, suggesting a critical function of PLCG1 in driving T cell activation. However, it remains unclear how these mutations regulate T cell physiology and pathology. Here, we investigate three common leukemia/lymphoma-associated mutations (R48W, S345F, and D1165H). We discover that these mutations induce hyperactive T cell signaling and cause pro-survival phenotypes. PLCG1 mutants enhance LAT condensation, calcium influx, and ERK activation. They also promote T cell proliferation, upregulate cell adhesion molecules, induce cell aggregation, and confer resistance to Vorinostat, an FDA-approved drug for cutaneous T-cell lymphoma. The resistance depends on ERK signaling and can be reversed with an ERK inhibitor. Interestingly, PLCG1 mutants also induce bystander drug resistance in nearby cells expressing wild-type PLCG1. Mechanistically, alpha smooth muscle actin, which is specifically induced by PLCG1 mutants, directly binds PLCG1 to promote its activation. These results demonstrate that hyperactive PLCG1 promotes T cell survival and drug resistance by inducing non-canonical signaling.