A FeCrMnNi concentrated solid-solution alloy was irradiated with a 2 MeV proton beam up to 1 dpa and 6 dpa at temperatures of 400 °C and 600 °C. The microstructural changes induced by irradiation were characterized using Transmission Electron Microscopy (TEM). In samples irradiated at 400 °C, Frank loops were the predominant form of lattice damage at 1 dpa, whereas small defect clusters were more prevalent at 6 dpa. For the sample irradiated to 1 dpa at 600 °C, both Frank loops and small defect clusters were present in similar density. Nanoindentation was employed to assess the changes in mechanical properties (hardness) post-irradiation, revealing significant hardening in all irradiated samples. The results indicated that the hardening effect began to saturate at 1 dpa or earlier. Additionally, nanoindentation creep tests with a 1200-second dwell period produced stress exponents comparable to those obtained from conventional creep testing. The findings suggest a shift in the deformation mechanism from dislocation glide to dislocation climb in the sample irradiated to 6 dpa at 400 °C.