Thermoelectric devices can directly convert thermal energy to electricity or vice versa with the efficiency being determined by the materials' dimension-less figure of merit (ZT). Since the revival of interests in the last decades, substantial achievements have been reached in search of high-performance thermoelectric materials, especially in the high temperature regime. In the near-room-temperature regime, MgAgSb-based materials are recently obtained with ZT approximate to 0.9 at 300 K and approximate to 1.4 at 525 K, as well as a record high energy conversion efficiency of 8.5%. However, the underlying mechanism responsible for the performance in this family of materials has been poorly understood. Here, based on structure refinements, scanning transmission electron microscopy (STEM), NMR experiments, and density function theory (DFT) calculations, unique silver and magnesium ion migrations in alpha-MgAg0.97Sb0.99 are disclosed. It is revealed that the local atomic disorders induced by concurrent ion migrations are the major origin of the low thermal conductivity and play an important role in the good ZT in MgAgSb-based materials.