The formation of thermally stable intermetallic phases plays a key role to enable high temperature applications for Mg alloys. In the ductile Mg-Al-Ca system, Ca-rich Laves phases are expected to be crucial for improving high temperature mechanical properties. However, the formation mechanisms of Ca-rich Laves phases are still unresolved. Here, we report atomic-scale experiments and simulations on transformation of Ca-rich Laves phases and formation of Ca-rich clusters as well as Ca-segregation behaviour in Mg-Al-Ca-Mn alloys in three conditions: as-cast, homogenised (500 °C, 20 h), and rolled (350 °C). The formation mechanisms of C36 Laves phase and Mg-rich particles (eutectic Mg) within C36 Laves phase were attributed to a divorced eutectic reaction. The transformation mechanisms from C36 to C15 Laves phase were revealed and discussed in terms of Mg out-diffusion of C36 Laves phase, the partitioning of Al into C36 Laves phase and the change of stacking sequences. The combined atomic-scale experimental and simulation investigation reveals complex interactions among various Ca-rich Laves phases in Mg-Al-Ca-Mn alloys.