Under compassionate use, chimeric antigen receptor (CAR) T cells have elicited durable remissions in patients with refractory idiopathic inflammatory myopathies (IIMs). Here, we report on the safety, efficacy, and correlative data of the first subject with the immune-mediated necrotizing myopathy (IMNM) subtype of IIM who received a fully human, 4-1BBz anti-CD19-CAR T cell therapy (CABA-201) in the RESET-Myositis phase I/II trial (NCT06154252). CABA-201 was well-tolerated following infusion. Notably, no evidence of cytokine release syndrome or immune effector cell-associated neurotoxicity syndrome was observed. Creatine kinase levels decreased, and muscular strength improved post-infusion. Peripheral B cells were depleted rapidly following infusion, and the subject achieved peripheral B cell aplasia by day 15 post-infusion. Peripheral B cells returned at 2 months post-infusion and were almost entirely transitional. Autoantibodies to SRP-9, SRP-72, SRP-54, and Ro-52, decreased relative to baseline, whereas antibodies associated with pathogens and vaccinations remained stable. The infusion product consisted of predominantly CD4+ effector memory T cells and exhibited in vitro cytolytic activity. Post-infusion, CABA-201 expansion peaked at day 15 and was preceded by a serum IFN-γ peak on day 8 with peaks in serum IL-12p40 and IP-10 on day 15. These data detail the safety, efficacy, and pharmacodynamics of CABA-201 in the first IMNM subject.