The long-term stability of structural materials in highly irradiated environments is one of the most important issues affecting the safe and economic operation of nuclear power plants. Since the macroscopic properties are related to the evolution of the microstructure, understanding the microstructure evolution under irradiation is essential to predict time-of-life of internals. In this work the influence of proton irradiation on the precipitation kinetics and the mechanical properties of PH 13-8 Mo maraging steel was investigated. Nanoindentation and micro-compression testing were used to determine radiation-induced hardening and the precipitation evolution was followed by atom probe tomography. Comparison of the solution annealed condition and a condition aged at 500°C for 2 h before and after irradiation showed that pre-existing precipitates delay material degradation due to irradiation. While nanoindentation showed a distinct hardness increase due to irradiation in the solution annealed sample, the hardness increased only slightly in the aged sample. Precipitates in the aged material are comparable in size and number density before and after irradiation, whereas their composition changed. It seems that a new precipitation type containing Ni, Al and Si developed. Also in the solution annealed and irradiated material decomposed zones enriched in Ni, Al and Si were found. They, in combination with radiation-induced defects, are most likely responsible for the hardness increase.