Nickel/carbon nanotube (Ni/CNT) composites with varying amounts of CNTs are deformed by high-pressure torsion (HPT) at different deformation temperatures to high strains, where no further refinement of the Ni matrix microstructure is observed. Mean Ni grain sizes increase with increasing HPT deformation temperature, while the size of the CNT agglomerates is significantly reduced. Additionally, the distribution of the agglomerates in the metal matrix becomes more homogenous. To investigate the mechanical performance of the HPT-deformed composites, uniaxial tensile and compression tests are conducted. Depending on the HPT deformation temperature and the resulting microstructure, either brittle or ductile fracture occurs, and the ultimate tensile strength varies between 900 and 2100 MPa. Increased HPT deformation temperatures induce a decrease in the anisotropy of the mechanical properties, mainly caused by a shrinking of the CNT agglomerates. It is shown, that tuning the HPT deformation temperature is the key for optimizing both the microstructure and the mechanical performance.