Over the past few decades, aluminium (Al) has been considered to be beneficial for castability but detrimental for the creep resistance of magnesium (Mg) alloys. However, an excellent creep resistance has been achieved in a die-cast Mg3.5RE(La,Ce,Nd)1.5GdMnAl alloy, with a super low steady-state creep rate (SCR) of 1.25×10-10 s-1 at 300°C/50MPa. Compared with the counterpart Al-free Mg3.5RE(La,Ce,Nd)1.5GdMn alloy, the SCR decreased by 73%. The synthetic effect of Al, Gd and Mn induced a novel thermally stable (TS) AlMnGd ternary shortrange order (SRO, 0-2nm)/cluster (2-10nm) in the Mg matrix. After creep at 300°C/50MPa for 400 hours, the AlMnGd SRO was still observed, and the AlMnGd clusters were under 10nm and coherent with the Mg matrix, without a unit cell structure. The AlMnGd SRO/clusters were massively observed for pinning dislocations, which was the main reason for the improvement in the creep resistance in contrast to the counterpart Al-free alloy. The TS Mg 12 RE(La,Ce,Nd) network at the grain boundaries (GBs) impeded dislocation mobility, which also played an important role for the creep resistance compared to the traditional die-cast Mg-Al-based alloys. Under the critical conditions of 300°C and 50-80MPa, the creep still satisfied the power law, and the dominant creep mechanisms were SRO/cluster drag dislocation gliding, associated with GB diffusion. This work provides clear evidence for the long-term existence of some SRO/clusters in Mg alloys under critically high temperatures and stresses. Moreover, the utilising of AlMnGd SRO/cluster can be a novel approach for designing heat-resistant Mg alloys with the sync boost of castability.