Following activation, naive T cells exit quiescence and require global translation for rapid expansion, yet the underlying mechanisms remain unclear. Here, we show that during T cell activation, cells upregulate the expression of N-acetyltransferase 10 (NAT10), an enzyme responsible for N-acetylcytidine (acC) modification of mRNAs. acC-modified Myc mRNAs show higher translation efficiency, enabling rapid synthesis of MYC protein and supporting robust T cell expansion. Conditional deletion of Nat10 in mouse T cells causes severe cell cycle arrest and limitation of cell expansion due to MYC deficiency, ultimately exacerbating infection in an acute lymphocytic choriomeningitis virus model. Additionally, T cells from older individuals with lower NAT10 levels show proliferative defects, which may partially account for impaired antiviral responses in older individuals. This study reveals a mechanism governing T cell expansion, signal-dependent mRNA degradation induction and the potential in vivo biological significance of acC modification in T cell-mediated immune responses.