Thermoelectric generators have a great potential in waste heat recovery and energy harvesting due to their principle of directly converting thermal into electrical energy. Despite a long history in space travel and a broad range of potential applications, TEGs are rarely found in terrestrial applications. Reasons are manufacturing problems and limited durability in dynamic operation conditions due to deficient mechanical properties of the thermoelectric materials, which often suffer from low strength and high brittleness. We present a concept for the basically independent tailoring of mechanical and thermoelectric properties. This can be achieved by the alloying of TE materials with additional elements having preferably no or only little influence on the TE properties. We demonstrate a route of improving the mechanical properties of PbTe by alloying with calcium. It is shown that calcium has minor effects on the thermoelectric properties of PbTe while significantly increasing hardness and fracture strength. As proof of concept, mechanically more stable sodium and calcium co-doped Pb0.966Ca0.02Na0.014Te with ZT exceeding 1.2 above 650 K is demonstrated.