Interest in geopolymers (GP) has been continuously increasing due to their comparable-to-concrete mechanical properties and enhanced durability characteristics. Despite their chemical stability at high temperatures, the brittle nature of geopolymers may prevent their use in applications requiring inelastic deformability, e.g., strengthening layers or elements subjected to dynamic loading. Strain-hardening geopolymer composites (SHGC) made with short high-performance fibers appear as a promising new class of materials that can yield a quasi-ductile tensile behavior under increasing loading. This article assesses distinct types of metakaolin-based SHGC with respect to the temperature effects on their mechanical performance. In particular, the efficiency of Na and K alkali solutions is compared when combined with short fibers made of polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE). Flexural properties were obtained for all material variations after exposure to a temperature of 100 °C or 200 °C in cooled down state (residual) and in hot state (quasi in-situ). Additionally, thermogravimetry (TGA), mercury intrusion porosimetry (MIP), dilatometry, and environmental scannning electron microscopy (ESEM) techniques were used. At room temperature, NaGP-based composites showed higher flexural strength due to the superior properties of the matrix and crack-bridging performance of the fibers. All quasi in-situ tested specimens demonstrated higher losses in strength and ductility when compared to the residual ones. This can be traced back to higher deformability of the fiber when in hot state. As opposed to PVA, the PE fibers yielded a higher thermal sensitivity in terms of crack-bridging capacity due to their lower melting temperature.