Abstract Micro-deformation testing has recently gained far-reaching scientific importance as it provides intrinsic information on the dynamics of plastic deformation which is concealed when bulk materials are tested. In this work, single-crystal Mg micropillars favorably oriented for mechanical twinning were tested in compression with concurrent scanning electron microscopy imaging. The experimental data were complemented by the finite element modeling in order to reveal the underlying physical background of the observed twinning dynamics. It was shown that the thickness of a twin should reach a critical value before triggering the nucleation of another twin to accommodate further strain. Nucleation and growth are repeated until the twins form throughout the whole micropillar, from top to bottom. Afterwards, the thickening and coalescence of all these twins take place until the entire micropillar volume is twinned. In addition, a line-by-line analysis of the scanning electron microscopy images was employed to reveal the twin lateral growth rates, which were shown to be on the order of 10−5–10−4 m/s.