Mutations in SCN1A, the gene encoding the brain voltage‐gated sodium channel α1 subunit (NaV1.1), are associated with genetic forms of epilepsy, including generalized epilepsy with febrile seizures plus (GEFS+ type 2), severe myoclonic epilepsy of infancy (SMEI) and related conditions. Several missense SCN1A mutations have been identified in probands affected by the syndrome of intractable childhood epilepsy with generalized tonic–clonic seizures (ICEGTC), which bears similarity to SMEI. To test whether ICEGTC arises from molecular mechanisms similar to those involved in SMEI, we characterized eight ICEGTC missense mutations by whole‐cell patch clamp recording of recombinant human SCN1A heterologously expressed in cultured mammalian cells. Two mutations (G979R and T1709I) were non‐functional. The remaining alleles (T808S, V983A, N1011I, V1611F, P1632S and F1808L) exhibited measurable sodium current, but had heterogeneous biophysical phenotypes. Mutant channels exhibited lower (V983A, N1011I and F1808L), greater (T808S) or similar (V1611F and P1632S) peak sodium current densities compared with wild‐type (WT) SCN1A. Three mutations (V1611F, P1632S and F1808L) displayed hyperpolarized conductance–voltage relationships, while V983A exhibited a strong depolarizing shift in the voltage dependence of activation. All mutants except T808S had hyperpolarized shifts in the voltage dependence of steady‐state channel availability. Three mutants (V1611F, P1632S and F1808L) exhibited persistent sodium current ranging from ∼1–3% of peak current amplitude that was significantly greater than WT‐SCN1A. Several mutants had impaired slow inactivation, with V983A showing the most prominent effect. Finally, all of the functional alleles exhibited reduced use‐dependent channel inhibition. In summary, SCN1A mutations associated with ICEGTC result in a wide spectrum of biophysical defects, including mild‐to‐moderate gating impairments, shifted voltage dependence and reduced use dependence. The constellation of biophysical abnormalities for some mutants is distinct from those previously observed for GEFS+ and SMEI, suggesting possible, but complex, genotype–phenotype correlations.