To enhance the performance and durability of cement-based materials under severe environmental conditions such as high temperatures and freeze-thaw cycles, the development of novel building materials with superior thermal resistance, frost resistance, and mechanical properties is of considerable significance. In this study, geopolymer cement mortar was prepared, and its performance was systematically compared with that of ordinary portland cement mortar in terms of early-age strength, shrinkage rate, water retention capacity, and bond strength. The performance variations after exposure to high temperature (800 °C) and freeze-thaw cycling were specifically investigated. Furthermore, the reinforcing effects of polypropylene fibers with different lengths on the mechanical properties of the mortar were analyzed. The results indicate that: 1) The geopolymer cement mortar outperforms ordinary mortar regarding early-age strength, 28 d shrinkage rate, water retention, and bond strength. 2) The material exhibits excellent high-temperature stability, retaining 61.6% of its compressive strength after exposure to 800 °C, which is significantly higher than the 25.5% retention rate of ordinary mortar. 3) After freeze-thaw cycling, the mass change rate and dynamic elastic modulus remain favorable, demonstrating outstanding frost resistance. 4) The addition of 15 mm polypropylene fibers significantly improves the flexural strength, with growth rates of 22.9% and 16.7% at 7 d and 28 d, respectively. The addition of 12 mm fibers yields the optimal enhancement in compressive strength, with growth rates of 12.1% and 9.5% at 7 d and 28 d, respectively.