In the present work, the correlation between microstructure and mechanical behaviour under quasi-static and dynamic loading of a novel designed high strength Fe-0.3Cr-0.4Mo-1.5Mn–3Ni-0.6C tool steel is presented. The alloy was manufactured under elevated cooling rates promoting a fine microstructure composed of martensite and retained austenite already in the as-cast state as detected via electron backscattered diffraction and X-ray diffraction. Due to this phase combination, a high compressive strength of about 3700 MPa combined with a total compressive strain of 24% can be observed as well as a pronounced work hardening. This behaviour was mainly traced back to deformation-induced martensite formation (transformation induced plasticity (TRIP) effect). Tests under dynamic loading conditions at a strain rate of 10² s⁻¹, which are also of high importance for later tool applications, display no significant change regarding the compressive strength and total deformation. Furthermore, an increased compressive yield strength and a pronounced work hardening could be detected under dynamic conditions, which was also attributed to the TRIP effect as for the quasi-static tests. Thereby, a decreasing rate of martensite formation with increasing deformation could be observed for both states. Furthermore, adiabatic shear bands (ASB) were detected in compression samples under dynamic loading via scanning electron microscopy and the failure behaviour was discussed with a focus on adiabatic shearing (also including transmission electron microscopy investigations). Concluding, the newly developed cast Fe-0.3Cr-0.4Mo-1.5Mn–3Ni-0.6C steel shows no significant strain rate sensitivity, which is an outstanding property of cast tool steels.