Magnesium alloy AE42 is a candidate material for high-temperature applications. However, as-cast AE42 alloy exhibits poor high-temperature creep properties because of the microstructure that shows a continuous network of β-Mg17Al12 and Al-RE precipitates along the boundaries of coarse α-Mg grains. In the current work, friction stir processing technique was adopted to refine the coarse as-cast structure of the AE42 alloy and break up the continuous network of β-Mg17Al12 and Al-RE precipitates. The creep properties of the parent material and friction-stir-processed specimens were determined by impression creep test at 150, 175, 200, 225 and 250 °C. The results demonstrated that the presence of fragmented precipitates and the elimination of continuous network structure improved the creep resistance of the AE42 alloy. The metallurgical analysis revealed that no new precipitates were formed after friction stir processing and creep testing. From the data, it can be concluded that friction stir processing of AE42 alloy can lead to an increase in service temperatures from 150 to 225 °C. © 2020, ASM International.