The distortion of the plasma boundary when three-dimensional resonant magnetic perturbations (RMPs) are applied has been measured in MAST H-mode plasmas. When the n = 3 RMPs are applied to control edge-localized modes (ELMs), the plasma experiences a strong toroidal corrugation. The displacement of the plasma boundary is measured at various toroidal locations and found to be of the order of 5% of the minor radius for an applied field magnitude which mitigates ELMs. The empirically observed corrugation of the plasma edge position agrees well with three-dimensional ideal plasma equilibrium reconstruction.