The atomic-scale homogenization of a face-centered-cubic-based high-entropy alloy (HEA), Al0.3Cu0.5CoCrFeNi, using severe plastic deformation (SPD) is reported. Atom probe tomography revealed that water quenching from high temperature cannot produce a homogeneous single phase, and clustering of Cu, Al and Ni still exists. Subsequent processing by high-pressure torsion produced nanostructured non-equilibrium single phase with homogeneous elemental distribution at atomic scale. Importantly, such a non-equilibrium single phase is stable at room temperature due to the sluggish diffusion kinetics. These observations suggest that SPD is an effective approach for producing single-phase HEAs for fundamental studies and applications. Display omitted •Conventional heat treatment cannot eliminate clustering in Al0.3Cu0.5CoCrFeNi HEA.•HPT enhances intermixing of elements in this multicomponent alloy.•Principal elements show nearly ideal random distribution at atomic scale after HPT.•Slow diffusion kinetics keeps the non-equilibrium alloy stable at room temperature.