Geopolymer concrete has attracted increasing attention as an alternative to ordinary Portland cement concrete because of its potential to reduce cement consumption and retain mechanical performance after thermal exposure. This study compared plain geopolymer concrete (GPC), steel fiber-reinforced geopolymer concrete (SFRGC), carbon fiber-reinforced geopolymer concrete (CFRGC), and hybrid fiber-reinforced geopolymer concrete (HFRGC) with ordinary Portland cement concrete (OPC). After 28 days of curing, the specimens were exposed to temperatures of 200, 400, 600, and 800 °C. Compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, stress–strain behavior, energy absorption capacity, mass loss, and microstructural changes were evaluated using SEM, XRD, and TGA analyses. At 800 °C, the residual compressive strength ratios of OPC, GPC, SFRGC, CFRGC, and HFRGC were 20.5%, 34.8%, 41.9%, 47.0%, and 42.7%, respectively. At the same temperature, the splitting tensile and flexural strengths of SFRGC were 234% and 306% higher than those of OPC, respectively, while the modulus of elasticity of CFRGC was 100.7% higher than that of OPC. The energy absorption capacity of HFRGC increased by 44% relative to its initial value. XRD results showed that the amorphous-phase indices of GPC and OPC at 800 °C were 0.62 and 0.43, respectively. Moreover, the total mass losses obtained from TGA were 9.22% for GPC and 10.12% for OPC. The results indicated that steel fibers were more effective in retaining tensile and flexural strengths, whereas carbon fibers provided greater retention of compressive strength and stiffness after thermal exposure.