Peripheral T cell diversity is virtually constant in the young, but is invariably reduced in aged mice and humans. CD8⁺ T cell clonal expansions (TCE) are the most drastic manifestation of, and possible contributors to, this reduced diversity. We show that the presence of TCE results in reduced CD8⁺, but not CD4⁺, T cell diversity, and in functional inability to mobilize parts of the CD8⁺ T cell repertoire affected by TCE. In the model of herpes simplex virus (HSV)-1 infection of B6 mice, >90% of the responding CD8⁺ T cells use Vβ10 or Vβ8 and are directed against a single glycoprotein B (gB_498-505) epitope, gB-8p. We found that old animals bearing CD8⁺ TCE within Vβ10 or Vβ8 families failed to mount an effective immune response against HSV-1, as judged by reduced numbers of peptide-major histocompatibility complex tetramer⁺ CD8 T cells and an absence of antiviral lytic function. Furthermore, Vβ8 TCE experimentally introduced into young mice resulted in lower resistance to viral challenge, whereas Vβ5⁺ TCE induced in a similar fashion did not impact viral resistance. These results demonstrate that age-related TCE functionally impair the efficacy of antiviral CD8⁺ T cell immunity in an antigen-specific manner, strongly suggesting that TCE are not the mere manifestation of, but are also a contributing factor to, the immunodeficiency of senescence.