The channel mobility in organic field-effect transistors that have a disordered semi-conducting layer in contact with an insulating polar dielectric material is calculated from the transconductance of the modelled transistor transfer response. The modelled transfer characteristics incorporate the additional energetic disorder that is introduced into the channel region from the polar gate dielectric, together with a carrier mobility enhancement factor due to localized state occupancy under charge accumulation conditions. A proposed mobility enhancement factor is independently determined from Monte Carlo simulations of carrier hopping through partially occupied energy landscapes that are subject to a Gaussian distribution of energetic disorder. The calculated field-effect mobility is found to be reduced relative to the bulk semiconductor mobility as the amount of energetic disorder in the channel region is increased through the use of more strongly polar gate dielectrics. The field-effect mobility is additionally found to decrease as the accumulated charge is confined to the most energetically disordered region in the vicinity of the insulator–semiconductor interface at higher gate voltages. The overall gate voltage dependence of the channel mobility is suppressed, however, by the mobility enhancement factor. Support for the model is found from recently published mobility data for a series of triarylamine field-effect devices.