Free surface roughening affects the tribological behavior of contacting bodies in multistage metal forming processes. This phenomenon was investigated on sheets of AISI 420 martensitic stainless steel by applying uniaxial and radial straining modes via tensile and Erichsen cupping tests, respectively. By means of focus variation microscopy, the free surface roughness evolution was studied for each sample at different values of strain. Moreover, the microscopic deformation of grains was visualized using scanning electron microscopy and differential interference contrast microscopy. The results show that the free surface roughening rate in the uniaxial strain mode is higher than that in the radial one. Furthermore, the surface height distribution of the deformed surfaces was found to be dependent on the deforming strain and tended toward a Gaussian one at higher strains. Grain boundaries and chromium carbide particles were found to be responsible for the peaks formed in the roughened free surfaces. Analyses of the deformed free surfaces showed distortion of the grain morphology, which is the mechanism responsible for free surface roughening.