The standard heat treatment of PH 13-8 Mo maraging steels consists of solution annealing and subsequent aging. Herein, it is investigated how an additional intercritical annealing step prior to aging affects the microstructure, and, consequently, the mechanical properties of a PH 13-8 Mo maraging steel. In situ techniques by means of high-temperature electron backscatter diffraction and high-temperature X-ray diffraction are applied to study the microstructural changes during intercritical annealing and subsequent aging. In addition, high-resolution investigation methods, such as transmission electron microscopy and atom probe tomography supplemented by transmission Kikuchi diffraction, are used for an in-depth characterization of the microstructure. The results reveal that a diffusion-controlled martensite to austenite transformation accompanied by partitioning of the substitutional atoms Cr, Ni, and Mo takes place during intercritical annealing. As a result of partitioning during intercritical annealing, an inhomogeneous distribution of Ni remains in the microstructure after the martensitic transformation. Consequently, the formation of reverted austenite is facilitated during subsequent aging due to existing Ni-enriched zones in martensite. Since the fracture toughness is significantly enhanced compared to the standard heat treatment, it is suggested that this improvement is related to the increased phase fraction of reverted austenite.