The effect of the manganese content (0–11%) on the transformation temperatures, the mechanical properties and microstructure development of five highly alloyed 14Cr–XMn–6Ni cast stainless steels with 0.1% nitrogen was studied. The examinations reveal that the Ms, As, and Af temperatures decrease with increasing manganese contents. As a result of low austenite stability, room temperature austenitic-martensitic as-cast microstructure was formed at manganese contents between 0 and 3%. At manganese levels of 6% and higher a fully austenitic as-cast microstructure was observed. The temperature dependence of tensile properties in alloys was explained on the basis of varying contributions to the strength and ductility of deformation-induced martensite and twin formation mechanisms. All investigated cast steel alloys achieved yield strengths above 200 MPa at room temperature. The increased proof stress is caused by solid solution strengthening due to the addition of nitrogen and manganese as well as phase hardening by as-quenched martensite.