We report the direct observation of a long-range field effect in devices, leading to large gate-induced changes of transport through crystals much thicker than the electrostatic screening length. The phenomenon—which manifests itself very differently from the conventional field effect—originates from the nonlocal nature of transport in the devices that are thinner than the carrier mean free path. We reproduce theoretically the gate dependence of the measured classical and quantum magnetotransport, and show that the phenomenon is caused by the gate tuning of the bulk carrier mobility by changing the scattering at the surface. Our results demonstrate experimentally the possibility to gate tune the electronic properties deep in the interior of conducting materials, avoiding limitations imposed by electrostatic screening.