Mechanical training (represented by cyclic loading and unloading) is frequently performed on Ni–Mn–Ga martensite to reduce undesirable variants for maximized shape memory performance. In this work, an effort was made to reveal the deformation mechanisms of 7M modulated (or 14M) martensite in a polycrystalline Ni–Mn–Ga alloy. It is demonstrated that during unidirectional compression, deformation bands form regularly and repeatedly through the sample. These bands are initiated from the colonies having martensite variants with high Schmid Factors (SFs) for the detwinning systems of the Type-I and II transformation twins (TrF-twins) and go through the colonies possessing variants with low SF for the detwinning systems of the TrF-twins. The deformation is realized by detwinning of the Type-I and Type-II TrF-twins in the high SF colonies but twinning and shuffling of the variants in the low SF colonies. The detwinning results in the thickening of the favorable variants, whereas the twinning gives rise to the formation of the new 7M variants and the shuffling induces intermartensitic transformation. Although these shear systems are activated individually, they act coordinately and form continuous strain paths through colonies, allowing the accommodation of the macroscopic deformation. The present results provide comprehensive information on the deformation mechanisms of Ni-Mn-Ga 7M martensite and their microstructural representation.