The technical potential of WCu alloys is limited by the modest fracture characteristics of the material system in its coarse-grained condition. To provide a nanocrystalline microstructure and improve mechanical properties, a W-50 at.% Cu composite was processed using high-pressure torsion deformation at a temperature of 200 ◦C. Therefore, two specimens were subjected to 100% and 1000% shear strains, respectively. Scanning electron and scanning transmission electron microscopy, including nanoscale energy dispersive X-ray spectroscopy mappings, were used to quantify the resulting microstructures. The average grain sizes for the 100% and 1000% deformed specimens were determined to be 14.7 ± 6.6 nm and 10.5 ± 5.6 nm, with the amount of mechanically intermixed W in the Cu grains increasing from 15.4 at.% to 15.9 at.%. X-ray diffraction and selected area electron diffraction studies both revealed strained lattice parameters of the W and Cu phases, respectively. Mechanical properties were investigated using in-situ notched microcantilever tests. The mean conditional fracture toughness and J-integral values were comparable for both conditions, at 3.7 ± 0.4 MPa√m and 245 ± 58 J/m2, respectively. The related behavior could be attributed to the low fault tolerance of the highly deformed states and was substantiated by cleaved globular W grains along the fractured surfaces. In addition, the detailed relationship between the altered grain boundary conditions, the degree of mechanical intermixing and the influence of the different microstructures on the fracture properties were carefully evaluated and discussed to pave the way for future application of these high-strength nanocomposites.