Ion irradiation has been used to investigate the radiation-induced precipitation of nanoclusters and changes in mechanical properties of commercial-grade T91 ferritic-martensitic steel irradiated with Fe ⁴⁺ ions up to 4.10 dpa at 301 – 311 °C. Atom probe tomography was used to analyse the microstructure segregation and cluster formation, while nanoindentation was used to measure the change in mechanical properties. At 0.12 dpa, Si/P-rich clusters formation is observed. At 1.76 dpa, Mn, Ni and Si -rich precipitates (MNSP) were observed with a composition range that is distinctly different than the typically cited G-phase. The MNSP number density was similar to that from prior neutron irradiation studies at similar temperatures. Segregation of such species to dislocation loops and lines was also discussed. The hardness increased with radiation dose up to measured 1.83. Comparisons were made between the observed microstructural and mechanical property changes as a function of ion irradiated dose. The use of APT and nanoindentation, when applied in tandem, have shown to provide an insight into how radiation-induced microstructural effects can explain the observed changes in mechanical properties.