Recent studies have indicated that doping, alloying, interface-engineering and nanostructuring are some of the strategies useful for obtaining high power factors and low thermal conductivities in materials that are needed for the fabrication of highly efficient thermoelectrics. With the intent of experimentally demonstrating the use of these strategies for designing highly efficient thermoelectrics, our group has in the past reported a solid-state phase transformation strategy for converting silicon nanowires into Mg2Si nanowires and Mg2Si welded nanowire networks. In this paper, the phase transformation strategy is extended to obtain Mg2Si0.92Sn0.08 nanowires from silicon nanowires. This report discusses not only the synthesis of Mg2Si0.92Sn0.08 nanowires from silicon nanowires, but also demonstrates that it is possible to control their diameters using variations of the silicon nanowire diameters as a parameter. Moreover, thermal conductivities of the nanowire assemblies discussed in detail in this paper indicated that nanostructuring through the formation of Mg2Si0.92Sn0.08 nanowires led to a drastic decrease in their thermal conductivities.