We calculate the electro-osmotic mobility and surface conductivity at a solid-liquid interface from a modified Poisson-Boltzmann equation, including spatial variations of the dielectric function and the viscosity that where extracted previously from molecular dynamics simulations of aqueous interfaces. The low-dielectric region directly at the interface leads to a substantially reduced surface capacitance. At the same time, ions accumulate into a highly condensed interfacial layer, leading to the well-known saturation of the electro-osmotic mobility at large surface charge density regardless of the hydrodynamic boundary conditions. The experimentally well-established apparent excess surface conductivity follows from our model for all hydrodynamic boundary conditions without additional assumptions. Our theory fits multiple published sets of experimental data on hydrophilic and hydrophobic surfaces with striking accuracy, using the nonelectrostatic ion-surface interaction as the only fitting parameter.