Lath martensite is widely present in advanced high strength steels as the key strengthening phase. Unexpectedly high ductility of lath martensite has been reported in the literature in both single-phase and multi-phase steels, however, without systematic identification of the underlying plasticity mechanisms. In this study, first, well-defined micro-tensile tests are carried out on fully martensitic steel with a clean large substructure and a variety of substructure boundary orientations with respect to the loading direction. Two deformation mechanisms of lath martensite were identified, namely, intra-lath crystallographic slip and apparent substructure boundary sliding, that compete with each other to carry the overall plasticity. The condition under which these two mechanisms are active has been clarified. It is found that boundary sliding is more easily activated than intra-lath crystallographic slip. In addition, for dual phase steel, as an example of multi-phase steels, the probability for sliding of lath martensite boundaries was estimated by boundary orientation characterization and micro-tensile tests. The results suggest that the apparent boundary sliding is also important for lath-martensite-containing multi-phase steels, which would explain prior reports in the literature of unexpectedly high local strains in the martensite regions.