Increasing the level of grain boundary (GB) segregation is one of the effective strategies for the preparation of new high-performance Mg alloys. Here based on careful experimental design, we clearly demonstrate that the addition of trace Sm could significantly increase the GB segregation concentration of Zn and Ca in the as-extruded dilute Mg–Zn–Ca–Mn alloy, and confirm that the increased GB segregation could improve the yield strength of the as-extruded alloy and further inhibit the grain growth and the separation of the segregation atmosphere from GBs during annealing. The proposed idea of multi-element promoting efficient GB segregation is expected to contribute to the design strategy of advanced Mg alloys.