This study aims to investigate the impact of moderate and elevated temperatures on compressive strength, mass loss, ultrasonic pulse velocity (UPV), and gas permeability of mortars made using metakaolin (MK) or Ordinary Portland cement (OPC). The geopolymer mortar comprises MK, activated by a solution of sodium hydroxide (SH) and sodium silicate (SS) with a weight ratio of SS/SH equal to 2.5. For most of the tests, the MK and OPC mortar specimens were cured for 7 and 28 days before exposure to elevated temperatures, ranging from 100 °C to 900 °C in increments of 100 °C. In the permeability tests, conducted at temperatures ranging from 100 °C to 300 °C in 50 °C increments, the results revealed significant findings. When exposed to 200 °C, MK geopolymer mortar demonstrated an increase in compressive strength by 83% and 37% for specimens initially cured for 7 and 28 days, respectively. A strong polynomial correlation between UPV and compressive strength in MK mortar was observed. Prior to heat exposure, the permeability of MK mortar was found to be four times lower than that of OPC mortar, and this difference persisted even after exposure to 250 °C. However, at 300 °C, the intrinsic permeability of MK mortar was measured at 0.96 mD, while OPC mortar exhibited 0.44 mD.