CD8+ T cells are indispensable for pathogen and tumour clearance, although can induce immunopathology if left unrestricted. T cell exhaustion, progressively programmed in the context of persistent antigen exposure, ensures partial control while mitigating immunopathological risk. However, exhaustion is characterised by curtailed proliferative capacity, cytokine production and cytotoxic functions, which tumours and chronic pathogens exploit to persist. As such, transiently disrupting exhaustion has emerged as a therapeutic strategy for treating cancer. Exhausted cells transition through a range of differentiation states, from stem-like progenitors that mediate response to checkpoint blockade, through to terminal effector or exhausted cells. Understanding the molecular pathways moderating the exhausted states is essential for augmenting cancer immunotherapy. We identify IKZF transcription factors, Ikaros (IKZF1), Helios (IKZF2) and Aiolos (IKZF3), as key mediators of T cell exhaustion, using gene knock-out mice and a CRISPR-mediated genetic knock-out system in both chronic infection and tumour models. IKZF1 and IKZF3 are critical regulators of exhausted subset expansion, retention, phenotype and cytokine function. Strikingly, dual IKZF1 and IKZF3 ablation boosts the response of anti-tumour and anti-viral exhausted T cells, potentially via the formation of atypical exhausted T cell populations and the enrichment of functional exhausted subsets. Cumulatively, our data highlight a novel role for IKZF transcription factors in biasing exhausted T cell differentiation and provides potentially important clinical implications, directing future cancer immunotherapies targeting T cell exhaustion.