The atomic topology of individual, highly mobile Type I and Type II twin boundaries in 10M Ni–Mn–Ga martensite was investigated by transmission electron microscopy (TEM). The twin boundaries established in a bulk single crystal showed twinning stresses of ∼1 MPa for Type I and ∼0.1 MPa for Type II. TEM lamellae with a (010) cross-section and the c-axis (easy-magnetization direction) lying in-plane were prepared by focused ion-beam milling, each containing a single twin boundary of specific type. High-resolution TEM confirmed an atomically sharp Type I twin boundary oriented along the rational (101) plane. The Type II boundary was also atomically sharp, apart from occasional single-atomic-plane steps, contrasting with previous suggestions of its diffuse nature. These findings provide the first direct atomic-scale comparison of both boundary types and offer critical structural insight into the origin of supermobility in Type II twin boundaries.