Neurons integrate and encode complex synaptic inputs into action potential outputs through a process termed "intrinsic excitability." Here, we report the essential contribution of fibroblast growth factor homologous factors (FHFs), a family of voltage-gated sodium channel binding proteins, to this process. Fhf1-/-Fhf4-/- mice suffer from severe ataxia and other neurological deficits. In mouse cerebellar slice recordings, WT granule neurons can be induced to fire action potentials repetitively (∼60 Hz), whereas Fhf1-/-Fhf4-/- neurons often fire only once and at an elevated voltage spike threshold. Sodium channels in Fhf1-/-Fhf4-/- granule neurons inactivate at more negative membrane potential, inactivate more rapidly, and are slower to recover from the inactivated state. Altered sodium channel physiology is sufficient to explain excitation deficits, as tested in a granule cell computer model. These findings offer a physiological mechanism underlying human spinocerebellar ataxia induced by Fhf4 mutation and suggest a broad role for FHFs in the control of excitability throughout the CNS.