Nanocomposite materials containing a soft and hard metal phase are a promising strategy to combine ultra-high strength, ductility and fracture toughness. However, given the rather brittle intercrystalline fracture mode, the true potential of these materials is only accessible after strengthening the vast number of interfaces within the composite. In this work, this is realized by doping a W–75Cu nanocomposite with either C, B, Hf or Re, elements that show promising effects on grain boundary cohesion in ab-initio calculations. The samples are fabricated from powders using severe plastic deformation and characterized using electron microscopy. Subsequently, various small-scale mechanical experiments are utilized to investigate the effect of the doping on strength, ductility and fracture toughness. While doping with C and B only leads to slight changes in mechanical properties, it was found that Hf increases the strength of the composite tremendously, most likely via the formation of nanosized oxides. Doping with Re showed an increase in strength and a major improvement in bending ductility, exhibiting “super-ductile” behavior in some cases. In microtensile tests this behavior was reduced, yet an increase in strength and ductility compared to the undoped composite was also apparent in these experiments. Interestingly enough, the fracture toughness of all doped variants did not change compared to the undoped W–Cu composite. This indicates that doping with Re improves resistance against crack initiation but not against crack propagation, making the materials properties highly sensitive to pre-existing defects and probed sample volume.