This study utilizes the Pandat software to design a novel ternary alloy, Mg-0.2Ce-0.2Ca (mass fraction, %). The Mg alloy samples are extruded conventionally and provide high strength and low alloying with yield strength of approximately 364 MPa and total content of only approximately 0.4%. The microstructures at different stages of extrusion are characterized, revealing the existence of twin in the Mg-0.2Ce-0.2Ca alloy throughout the extrusion process, indicating high twin migration resistance. In the middle and later stages of extrusion, dynamically recrystallized grains nucleate at regions of intersected twinning variants, leading to a significant reduction in the proportion of twinning interfaces. Moreover, during the early stage of extrusion, a large number of <c + a> dislocations are stored in the Mg-0.2Ce-0.2Ca alloy, and the dislocation-dominated recovery/recrystallization mechanism is functional until the late stage of extrusion due to the high slipping resistance of dislocations. This mechanism directly contributes to the formation of ultrafine grains in present Mg alloy. The results show that the addition of Ca increases the resistance of twinning motion in the Mg matrix, while the addition of Ce and Ca induces multisystem slip, which are the main mechanisms for regulating the microstructure evolution of Mg-Ce-Ca alloy during extrusion. These findings have significant implications for the development of new high-strength, low-alloyed Mg alloys.