Alumina aerogels are desirable for lightweight and highly efficient thermal insulation. However, they are typically constrained by brittleness and structural collapse at high temperatures. The manufacture of alumina aerogels with ultralow thermal conductivity and excellent thermal stability at high temperatures beyond 1300 °C is still challenging. Herein, alumina aerogels with superior ultrahigh-temperature-resistant and thermal insulation were successfully prepared by assembling the α-Al2O3 nanosheets with silica sols as the high-temperature binders. Benefiting from the generation of the mullite-covered alumina biphasic structure, the α-Al2O3 nanosheet-based aerogels (ANSAs) exhibit excellent thermal and chemical stabilities even after calcination at as high as 1600 °C. The ANSAs had a low thermal conductivity (0.029 W·m–1·K–1 at room temperature), structural stability with a measured compressive strength of 0.6 MPa, and good thermal shock resistance. Furthermore, the 2D α-alumina@mullite core–shell sheets were also prepared as assembly units to construct aerogels (AMSAs). This core–shell structure can improve temperature resistance through inter-lattice suppression under continuous energy input at high temperatures. The AMSAs have a linear shrinkage of only 2.7% after calcination at 1600 °C for 30 min, further improving the temperature resistance, making them an ideal super-insulating material for applications at extremely high temperatures.