The effects of welding and post-weld heat treatment (PWHT) on nanoscale coprecipitation, grain structure, and mechanical properties of an ultra-high strength steel were studied through a combination of atom probe tomography (APT) and mechanical tests. Our results indicate that the welding process dissolves all pre-existing nanoparticles and causes grain coarsening in the fusion zone, resulting in a soft and ductile weld without any cracks in the aswelded condition. A 550 °C PWHT induces fine-scale re-precipitation of NiAl and Cu coprecipitates with high number densities and ultra-fine sizes, leading to a large recovery of strength but a loss of ductility with intergranular failure, whereas a 600 °C PWHT gives rise to coarse-scale re-precipitation of nanoparticles together with the formation of a small amount of reverted austenite, resulting in greater recovery in both strength and ductility. Our analysis indicates that the degree of strength recovery is dependent mainly upon the re-precipitation