Nano-scaled one-dimensional (1D) metallic materials show tremendous potential in various important technological applications, such as flexible transparent electrodes and catalysis. However, achieving scalable preparation of nanoscale 1D metallic materials remains a significant challenge for most metallic compositions. The phenomenon of A-site whisker growth from MAX phases provides an opportunity for innovative 1D metal fabrication techniques. However, the whisker diameters derived from MAX phases are often too large to satisfy the requirements of advanced technologies. Herein, we introduce Sn nanocrystals with different particle sizes as the initial seeds to tune the diameter of Sn whiskers, fed by the atomized tin resulted from the mechanochemical decomposition of Ti2SnC. It was found that incorporating nano-scaled (50–300 nm) and micro-scaled (0.5–1 μm) Sn crystal seeds resulted in the growth of whiskers with diameters matching the respective seed sizes, and the reduced melting point phenomenon of the nano-sized Sn wires conformed to the size-dependent behavior of metal nanowires. The diverse A-site elements within MAX phases implied that this method could be a generic strategy for synthesizing various nanoscale 1D metallic materials with controllable diameters.