Precipitation hardened steels are widely used in various engineering fields due to their high strength, high fracture toughness, good ductility and ease of machinability. As two kinds of common precipitates, Cu-rich and NiAl phases play an important role on the mechanical properties of steels. The obvious effects of Mn on the precipitate evolution of Cu-rich phase and NiAl phase in steel have been observed respectively. However, the effect of Mn is complex, when Cu-rich phase and NiAl phase exist at the same time. This work aims to reveal the effects of Mn on the co-precipitation of Cu-rich phase and NiAl phase in steel. Fe-Cu-Ni-Al and Fe-Cu-Ni-Al-Mn steels were aged at 500 ℃ for different times after solution treatment at 900 ℃ for 2 h. Hardness testing indicates that by adding 2.17%Mn, Fe-Cu-Ni-Al-Mn steel shows a peak hardness of 420 HV which is 80 HV higher than Fe-Cu-Ni-Al steel (about 340 HV). And Fe-Cu-Ni-Al-Mn steel reaches the peak hardness at 1 h which is 1 h earlier as compared with Fe-Cu-Ni-Al steel at 2 h. Moreover, the peak hardness plateau of Fe-Cu-Ni-Al-Mn steel only lasts for 7 h which is far less than that of Fe-Cu-Ni-Al steel. All in all, the addition of Mn enhances the effect of precipitation hardening at early aging, and accelerates the whole process of precipitation hardening. Atom probe tomography (APT) results reveal that Mn increases the nucleation rate of precipitates at early ageing, accelerates the growing and coarsening of precipitates and then accelerates the separation of the Cu-rich phase and NiAl phase. This is due to Mn can reduce the energy for nucleation and accelerate the diffusion rate of elements in the matrix, while the partial substitution of Mn for Al in the NiAl phase can form point defects which can accelerate the diffusion rate of Cu in NiAl phase.