The creep behavior of T91 steel at ultrahigh creep temperatures (650–750 °C) at 100 MPa was investigated using small-size specimens, and the related microstructures were characterized. The results showed that the creep failure susceptibility of T91 steel increases significantly as the creep temperature increases from 650 °C to 675 °C. The creep life rapidly decreased from 7880 h to 224 h, and the fracture surface shrinkage increased from 61.9% to 86.5%. Thermodynamic calculations and microstructure observations showed that the increase in creep temperature has a significant effect on the morphology of M₂₃C₆-type carbides, which were distributed around grain boundaries, significantly pinning and hindering the movement of dislocations and the grain boundaries. As the creep temperature increases, the subcrystalline and M₂₃C₆-type carbides significantly coarsen, which is the main reason for the rapidly shortening creep life of T91 steel. This research provides guidance for the material design and microstructure regulation of T91 steel in service at ultrahigh environmental temperatures.