The effects of artificial aging (T6) on the creep resistance with tensile stresses in the range of 50−80 MPa at 175 °C were investigated for an extruded Mg−1.22Al−0.31Ca−0.44Mn (wt.%) alloy. The Guinier-Preston (G.P.) zones primarily precipitate in the sample aged at 200 °C for 1 h (T6-200°C/1h), while the Al₂Ca phases mainly precipitate in the sample aged at 275 °C for 8 h (T6-275°C/8h). The T6-200°C/1h sample exhibits excellent creep resistance, with a steady-state creep rate one order of magnitude lower than that of the T6-275°C/8h sample. The abnormally high stress exponent (~8.2) observed in the T6-200°C/1h sample is associated with the power-law breakdown mechanism. TEM analysis illuminates that the creep mechanism for the T6-200°C/1h sample is cross-slip between basal and prismatic dislocations, while the T6-275°C/8h sample exhibits a mixed mechanism of dislocation cross-slip and climb. Compared with the Al₂Ca phase, the dense G.P. zones effectively impede dislocation climb and glide during the creep process, demonstrating superior creep resistance of the T6-200°C/1h sample.