We demonstrate that perfectly reproducible and homogeneous core-shell Si1−xGex/Ge nanowires can be produced by a two step nanofabrication process. The process makes use of a combination of Liquid Metal Alloy Ion Source–Focused Ion Beam (LMAIS-FIB) nanomilling and condensation. In a first step, we fabricate arrays of SiGe wires by LMAIS-FIB milling of fully relaxed Si1−xGex pseudo-substrates. The use of Ge2+ ions during this step avoids any metallic contamination of the nanowires. In a second step, we both reduce the diameter of the wires and form the core-shell configuration by oxido-reduction of the wires. Large arrays of core-shell nanowires with extended aspect ratio (length over diameter), small diameters and ultra-thin shell thickness are fabricated. Multilayer core-shell configurations with tunable arrangements could also be produced by repeated condensation cycles.