The mechanical response, on a microscopic and macroscopic level, and the deformation-induced martensitic transformation (DIMT) were investigated in multi-phase medium Mn steels (MMnS) with 6, 8 and 9 wt% Mn using in situ high-energy synchrotron x-ray diffraction during tensile loading. Prior to the in-situ analysis, a similar heat treatment finishing with an intercritical annealing was imposed on all MMnS. The initial microstructure prior to tensile loading was investigated by electron backscatter diffraction analysis. The volume fraction of austenite (gamma) after the heat treatment decreases from 60.2% to 50.7%, and 23.6% with increasing Mn content from 6 to 8 and 9 wt% Mn, respectively. This is mainly due to the difference in the formation of athermal alpha '-martensite. Athermal epsilon-martensite also formed in the MMnS with 8 and 9 wt% Mn, whereas no athermal epsilon-martensite formed in the MMnS with 6 wt% Mn. The alloys have quite different deformation behavior due to the different microstructures, and the majority of the load is carried by the phase that forms a continuous network throughout the steel, which in turn influences the DIMT. These results reveal the importance of assessing both phase-specific strain/stress and the inherent mechanical stability of the austenite in order to predict the macroscopic mechanical properties of the steel. As an example, this is witnessed by the comparison of MMnS9 and MMnS8. Austenite in MMnS9 bears about half the load as compared to austenite in MMnS8 during early deformation due to a continuous network of athermal alpha '-martensite resulting in significant load partitioning from austenite to alpha '-martensite. Thus, the mechanical driving force for DIMT in MMnS9 is reduced and therefore causes lower DIMT kinetics in MMnS9 than in MMnS8, even though MMnS9 has lower inherent austenite stability.