Understanding immunological memory formation depends on elucidating how multipotent memory precursor (MP) cells maintain developmental plasticity and longevity to provide long-term immunity while other effector cells develop into terminally differentiated effector (TE) cells with limited survival. Profiling active (H3K27ac) and repressed (H3K27me3) chromatin in naive, MP, and TE CD8⁺ T cells during viral infection revealed increased H3K27me3 deposition at numerous pro-memory and pro-survival genes in TE relative to MP cells, indicative of fate restriction, but permissive chromatin at both pro-memory and pro-effector genes in MP cells, indicative of multipotency. Polycomb repressive complex 2 deficiency impaired clonal expansion and TE cell differentiation, but minimally impacted CD8⁺ memory T cell maturation. Abundant H3K27me3 deposition at pro-memory genes occurred late during TE cell development, probably from diminished transcription factor FOXO1 expression. These results outline a temporal model for loss of memory cell potential through selective epigenetic silencing of pro-memory genes in effector T cells.